COMPARATOR, AD CONVERTER, PHOTOELECTRIC CONVERSION DEVICE, IMAGING SYSTEM, AND MOBILE OBJECT
A disclosed comparator includes a comparison circuit including a differential unit that compares an input signal with a reference signal and changes a level of a signal output to a first node in accordance with a result of comparison and an amplifier unit that includes a load element and outputs a signal in accordance with a potential of the first node to a second node, and a positive feedback circuit that is connected to the second node and a third node and changes a level of a signal at the third node at a rate higher than a change rate of a level of a signal at the second node in accordance with a change in a level of a signal at the second node.
The present invention relates to a comparator, an AD converter, a photoelectric conversion device, an imaging system, and a movable object.
Description of the Related ArtAn imaging device on which a column-parallel analog-to-digital (AD) converter including AD conversion units on a pixel column basis is mounted is proposed. A typical column-parallel AD converter performs AD conversion on a pixel signal by using a comparator to compare a pixel signal with a reference signal whose level changes with time and counting the time from start of the comparison to inversion of the output signal of the comparator. International Publication No. WO2018/037901 discloses an imaging device including a comparator including a positive feedback circuit used for increasing the transition rate when an output signal is inverted.
In the conventional comparator, however, a malfunction may occur due to variation of element characteristics or the like. Thus, when an AD conversion circuit is formed using such a comparator, an AD conversion error may occur.
SUMMARY OF THE INVENTIONThe object of the present invention is to provide a comparator that may suppress a malfunction due to variation of element characteristics or the like and an AD converter, a photoelectric conversion device, and an imaging system using such a comparator.
According to one aspect of the present invention, provided is a comparator including a comparison circuit including a differential unit that compares an input signal with a reference signal and changes a level of a signal output to a first node in accordance with a result of comparison and an amplifier unit that includes a load element and outputs a signal in accordance with a potential of the first node to a second node, and a positive feedback circuit that is connected to the second node and a third node and changes a level of a signal at the third node at a rate higher than a change rate of a level of a signal at the second node in accordance with a change in a level of a signal at the second node.
Further, according to another aspect of the present invention, provided is an analog-to-digital converter including the comparator described above, and a counter circuit that outputs, as digital data of the input signal, a count value in accordance with a length of a period from start of comparison between the input signal and the reference signal to a change of a level of the signal output to the first node.
Further, according to yet another aspect of the present invention, provided is a photoelectric conversion device including a plurality of pixels arranged so as to form a plurality of rows and a plurality of columns and each including a photoelectric converter, a plurality of output lines each arranged on each of the plurality of columns and each connected to pixels on a corresponding column, and a plurality of analog-to-digital converters described above each connected to each of the plurality of output lines and configured to perform analog-to-digital conversion on pixel signals output from the pixels on a corresponding column.
Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.
Preferred embodiments of the present invention will now be described in detail in accordance with the accompanying drawings.
First EmbodimentThe general configuration of a photoelectric conversion device according to a first embodiment of the present invention will be described with reference to
As illustrated in
In the pixel array unit 10, a plurality of pixels 12 arranged in a matrix over a plurality of rows and a plurality of columns are provided. Although
On each row of the pixel array unit 10, a control line 14 is arranged extending in a first direction (horizontal direction in
On each column of the pixel array unit 10, an output line 16 is arranged extending in a second direction (vertical direction in
The vertical scanning circuit 20 is a control circuit unit that supplies control signals to the pixels 12 via the control line 14 provided on each row of the pixel array unit 10, and the control signals are used for driving readout circuits in the pixels 12 when reading out signals from the pixels 12. The vertical scanning circuit 20 may be formed using a shift register or an address decoder.
The AD conversion circuit unit 30 includes a plurality of comparators 32 and a plurality of pulse generators 46 in association with each column of the pixel array unit 10. Each of the plurality of comparators 32 includes a comparison circuit 34 and a positive feedback circuit 40. The comparison circuit 34 is formed of a differential pair circuit, for example, and has two input nodes (a non-inverting input terminal (+) and an inverting input terminal (−)) and one output node. The non-inverting input terminal of the comparison circuit 34 is connected to the output line 16 on a corresponding column. The inverting input terminal of the comparison circuit 34 is connected to a reference signal generation circuit 48 via a reference signal line 48a common to respective columns. The output node of the comparison circuit 34 is connected to the input node of the positive feedback circuit 40. The output node of the positive feedback circuit 40 is connected to the input node of the pulse generator 46. A known one-shot pulse generation circuit that outputs one shot of pulse upon detecting a rising edge or a falling edge of an input signal is applicable to the pulse generator 46. Note that the specific configuration and operation of the comparator 32 will be described later.
The reference signal generation circuit 48 supplies a reference signal having a predetermined amplitude to the comparator 32 on each column via the reference signal line 48a. The reference signal may be a signal whose signal level (magnitude of the signal) changes with time, for example. Typically, the reference signal is a ramp signal. The ramp signal is a signal whose signal level monotonically changes with time, which is a signal whose output voltage monotonically decreases or monotonically increases with time, for example. Note that the reference signal is not particularly limited as long as it has an amplitude that is applicable to AD conversion. The operation of the reference signal generation circuit 48 is controlled by the control circuit 80.
The memory unit 50 includes a plurality of memories 52 in association with each column of the pixel array unit 10. Each of the plurality of memories 52 may include an N-memory (not illustrated) that holds digital data for a noise signal and an S-memory (not illustrated) that holds digital data for a light detection signal. Each of the plurality of memories 52 has two input nodes, one output node, and one control node. One of the input nodes of the memory 52 is connected to the output node of the pulse generator 46 on a corresponding column. The other input node of the memory 52 is connected to the counter circuit 54 via a count signal line 54a common to respective columns. The output node of the memory 52 is connected to a horizontal output line 56. The control node of the memory 52 is connected to the horizontal scanning circuit 60.
The counter circuit 54 starts a count operation in synchronization with a timing of start of a change in a signal level of a reference signal output from the reference signal generation circuit 48 and outputs a count signal indicating the count value to the memory 52 on each column.
The horizontal scanning circuit 60 is a circuit unit that supplies control signals to the memories 52 on respective columns sequentially on a column basis, and the control signals are used for outputting pixel signals stored in the memories 52 on respective columns. Control lines of the horizontal scanning circuit 60 provided in association with respective columns of the pixel array unit 10 are connected to the memories 52 on corresponding columns. In response to receiving a control signal via the control line on a corresponding column of the horizontal scanning circuit 60, the memory 52 on each column outputs the held pixel signal to the output circuit 70 via the horizontal output line 56.
The output circuit 70 is a circuit unit that performs predetermined signal processing on a signal on a column selected by the horizontal scanning circuit 60 and outputs the signal to the external of the photoelectric conversion device 100. The output circuit 70 includes a signal processing unit formed of a buffer amplifier, a differential amplifier, or the like and performs signal processing such as an amplification process, a digital correlated double sampling (CDS) process, or the like. The digital CDS process is signal processing that performs a differential process of (S−N) when the memory 52 stores two types of digital data, namely, a noise signal N and a light detection signal S as a pixel signal. Further, the output circuit 70 further includes an external interface such as Low Voltage Differential Signaling (LVDS) and outputs a digital signal obtained after signal processing to the external of the photoelectric conversion device 100.
The control circuit 80 is a circuit unit used for supplying control signals to the vertical scanning circuit 20, the AD conversion circuit unit 30, the reference signal generation circuit 48, the memory unit 50, the counter circuit 54, and the horizontal scanning circuit 60, and the control signals are used for controlling the operations of the above components and the timings thereof. Some or all of the control signals supplied to vertical scanning circuit 20, the AD conversion circuit unit 30, the reference signal generation circuit 48, the memory unit 50, the counter circuit 54, and the horizontal scanning circuit 60 may be supplied from the external of the photoelectric conversion device 100.
Next, a configuration example of a pixel in the photoelectric conversion device according to the present embodiment will be described with reference to
Each of the pixels 12 may be formed of a photoelectric converter PD, a transfer transistor M1, a reset transistor M2, an amplifier transistor M3, and a select transistor M4, as illustrated in
The photoelectric converter PD is a photodiode, for example, the anode thereof is connected to a reference voltage node, and the cathode thereof is connected to the source of the transfer transistor M1. The drain of the transfer transistor M1 is connected to the source of the reset transistor M2 and the gate of the amplifier transistor M3. The connection node of the drain of the transfer transistor M1, the source of the reset transistor M2, and the gate of the amplifier transistor M3 is a so-called floating diffusion portion FD. The floating diffusion portion FD includes a capacitance component (floating diffusion capacitance) and has a function as a charge holding portion.
The drain of the reset transistor M2 and the drain of the amplifier transistor M3 are connected to the power supply voltage node to which a power supply voltage VDD is supplied. The source of the amplifier transistor M3 is connected to the drain of the select transistor M4. The source of the select transistor M4 is connected to the output line 16. The output line 16 is connected to the current source 18.
Note that the pixel 12 is not necessarily required to have the select transistor M4 and may have a pixel configuration not including the select transistor M4. In such a case, the source of the amplifier transistor M3 is connected to the output line 16.
In the case of the pixel configuration of
A control signal ΦTX that is a drive pulse used for controlling the transfer transistor M1 is supplied to the signal line TX from the vertical scanning circuit 20. A control signal ΦRES that is a drive pulse used for controlling the reset transistor M2 is supplied to the signal line RES from the vertical scanning circuit 20. A control signal ΦSEL that is a drive pulse used for controlling the select transistor M4 is supplied to the signal line SEL from the vertical scanning circuit 20. When each transistor is formed of an n-channel transistor, a corresponding transistor is turned on in response to a control signal at a High level (hereafter, denoted as “H level”) being supplied from the vertical scanning circuit 20. Further, a corresponding transistor is turned off in response to a control signal at a Low level (hereafter, denoted as “L level”) being supplied from the vertical scanning circuit 20.
Next, the overview of the operation of the imaging device according to the present embodiment will be described with reference to
Once light enters the pixel array unit 10, the photoelectric converter PD of each pixel 12 converts (photoelectrically converts) the incident light into an amount of charge in accordance with the light amount and accumulates the generated charge. When turned on, the transfer transistor M1 transfers charge held by the photoelectric converter PD to the floating diffusion portion FD. The floating diffusion portion FD holds charge transferred from the photoelectric converter PD and has a voltage in accordance with the amount of the charge transferred from the photoelectric converter PD due to charge-voltage conversion caused by the capacitance of the floating diffusion portion FD.
The amplifier transistor M3 is configured such that the power supply voltage VDD is supplied to the drain and bias current is supplied to the source from the current source 18 via the select transistor M4 and forms an amplifier unit (source follower circuit) whose gate is the input node. Accordingly, the amplifier transistor M3 outputs a signal based on the voltage of the floating diffusion portion FD to the output line 16 via the select transistor M4. In this sense, the amplifier transistor M3 also serves as an output unit that outputs a pixel signal based on charge held by the floating diffusion portion FD. When turned on, the reset transistor M2 resets the floating diffusion portion FD to a voltage in accordance with the power supply voltage VDD. Note that, in this specification, a signal output to the output line 16 when the floating diffusion portion FD is in a reset state is referred to as a noise signal, and a signal output to the output line 16 in a state where the floating diffusion portion FD holds signal charge is referred to as a light detection signal.
The transfer transistor M1, the reset transistor M2, and the select transistor M4 of the pixel 12 are controlled on a row basis by the control signals ΦTX, ΦRES, and ΦSEL supplied from the vertical scanning circuit 20, as described above. Pixel signals of the pixels 12 belonging to a row selected by the control signal ΦSEL are simultaneously output to the output line 16 which respective pixels 12 correspond to.
The comparator 32 on each column compares the level of a pixel signal read out from the pixel on a corresponding column via the output line 16 with the reference signal supplied from the reference signal generation circuit 48 and outputs a signal in accordance with a result of the comparison to the pulse generator 46. Specifically, the comparator 32 compares the magnitude of a pixel signal (signal SIG) with the magnitude of the reference signal (reference signal RAMP) and changes the level of the output signal at a timing when the level relationship between these signals is inverted. The pulse generator 46 outputs one shot of pulse in response to a change of the level of the output signal of the comparator 32.
The counter circuit 54 starts count from the start of comparison of a pixel signal with the reference signal and outputs a count signal indicating a count value to the memory 52 on each column via the count signal line 54a. The memory 52 on each column stores, as digital data of the pixel signal, a count value indicated by a count signal being output from the counter circuit 54 at a timing when a pulse is output from the pulse generator 46 on a corresponding column. This count value corresponds to the length of a period from the start of comparison of a pixel signal with the reference signal to a change of the level of the output signal. In such a way, it is possible to digitally convert an analog signal output from the pixel 12 into a digital signal.
The horizontal scanning circuit 60 supplies control signals to the memories 52 on respective column of the memory unit 50 sequentially on a column basis. The memory 52 on a column that receives a control signal from the horizontal scanning circuit 60 outputs a held pixel signal to the horizontal output line 56. The output circuit 70 performs predetermined signal processing on a pixel signal output from the memory unit 50 and outputs the pixel signal to the external of the photoelectric conversion device 100.
Next, the configuration example of the comparator 32 in the photoelectric conversion device according to the present embodiment will be described with reference to
The comparator 32 is formed including the comparison circuit 34 and the positive feedback circuit 40, as described above. Although the comparator 32 and the pulse generator 46 are described as separate components in the present embodiment, the pulse generator 46 may be configured as a part of the comparator 32.
The comparison circuit 34 may be formed of p-channel transistors MP1, MP2, and MP3, n-channel transistors MN1 and MN2, a tail current source 36, and a current source 38, as illustrated in
The source of the p-channel transistor MP1, the source of the p-channel transistor MP2, and the source of the p-channel transistor MP3 are connected to the power supply voltage node. The drain of the p-channel transistor MP1 is connected to the drain of the n-channel transistor MN1. The drain of the p-channel transistor MP2 is connected to the drain of the n-channel transistor MN2. The gate of the p-channel transistor MP1 and the gate of the p-channel transistor MP2 are connected to the connection node between the drain of the p-channel transistor MP1 and the drain of the n-channel transistor MN1. The tail current source 36 is connected between each source of the n-channel transistor MN1 and the n-channel transistor MN2 and the reference voltage node (for example, a ground node). The connection node between the drain of the p-channel transistor MP2 and the drain of the n-channel transistor MN2 is connected to the gate of the p-channel transistor MP3. That is, the connection node between the drain of the p-channel transistor MP2 and the drain of the n-channel transistor MN2 is the output node of the differential unit 34a, and the gate of the p-channel transistor MP3 is the input node of the amplifier unit 34b. The current source 38 is connected between the drain of the p-channel transistor MP3 and the reference voltage node.
Accordingly, the n-channel transistor MN1 and the n-channel transistor MN2 form a differential pair, and the p-channel transistor MP1 and the p-channel transistor MP2 form a current mirror. The gate of the n-channel transistor MN2 is the non-inverting input terminal of the comparison circuit 34 to which the signal SIG of the output line 16 is supplied. The gate of the n-channel transistor MN1 is the inverting input terminal of the comparison circuit 34 to which a reference signal RAMP is supplied from the reference signal line 48a. The connection node between the drain of the p-channel transistor MP3 and the current source 38 is the output node (node FBIN) of the comparison circuit 34.
The current source 38 forms a load element of the amplifier unit 34b, typically, which may be formed of the n-channel transistor MN6 that forms a gate grounded circuit and whose gate is supplied with a bias voltage VB that is different from the reference voltage, as illustrated in
Alternatively, the current source 38 may also be formed of a circuit including the n-channel transistor MN6, a bias hold capacitor Cbh, and switches SW2 and SW3, as illustrated in
The bias voltage VB used for causing predetermined current Ito flow in the n-channel transistor MN6 is supplied to the gate of the n-channel transistor MN6 via the switch SW3. By once turning on and then turning off the switch SW3, it is possible to hold the bias voltage VB in the bias hold capacitor Cbh. With such a configuration, even if the reference voltage GND fluctuates, it is possible to maintain the gate-source voltage Vgs of the n-channel transistor MN6 at a constant voltage and suppress the current I from fluctuating. Further, by turning off the switch SW2, it is possible to reduce power consumption when not used.
Alternatively, as the load element 38a of the amplifier unit 34b, a passive load such as a resistor R1 may be used instead of an active load such as the current source 38, as illustrated in
The positive feedback circuit 40 has an input node and an output node. The positive feedback circuit 40 is a circuit that changes a signal level of the output node at a rate higher than the changing rate of the signal level of the input node in response to a change in the signal level of the input node. In the following description, changing the signal level of the output node at a rate higher than the changing rate of the signal level of the input node may be expressed as “facilitating” a change of the signal level of the output node. The positive feedback circuit 40 may be formed of the p-channel transistor MP4, MP5, and MP6 and the n-channel transistor MN3 as illustrated in
The source of the p-channel transistor MP5 and the source of the p-channel transistor MP6 are connected to the power supply voltage node. The drain of the p-channel transistor MP5 is connected to the source of the p-channel transistor MP4. The drain of the p-channel transistor MP6 is connected to the drain of the n-channel transistor MN3. The drain of the p-channel transistor MP4 is connected to the gate of the p-channel transistor MP6 and the gate of the n-channel transistor MN3. The source of the n-channel transistor MN3 is connected to the reference voltage node. The gate of the p-channel transistor MP4 is connected to the connection node between the drain of the p-channel transistor MP6 and the drain of the n-channel transistor MN3.
The connection node (node FBIN) of the drain of the p-channel transistor MP4, the gate of the p-channel transistor MP6, and the gate of the n-channel transistor MN3 is the input node of the positive feedback circuit 40. The input node of the positive feedback circuit 40 is connected to the connection node of the p-channel transistor MP3 to the current source 38, that is, the output node of the comparison circuit 34. The connection node of the gate of the p-channel transistor MP4, the drain of the p-channel transistor MP6, and the drain of the n-channel transistor MN3 is the output node (node OUT) of the positive feedback circuit 40. The p-channel transistor MP5 is controlled by a control signal INI supplied from the control circuit 80.
Note that the source and the drain of a transistor may be called opposite names in accordance with the conductivity type thereof, a function of interest, or the like. In this specification, the source and the drain may be denoted as a main node without distinction, and the gate may be denoted as a control node.
Next, the operation of the comparator 32 in the photoelectric conversion device according to the present embodiment will be described with reference to
At time t0, a signal (noise signal) in accordance with the reset level of the pixel 12 is being output as the signal SIG to the output line 16. Further, the reference level of the reference signal RAMP whose change in the potential level has not yet been started is higher than the level of the signal SIG in accordance with the reset level of the pixel 12. That is, in the present embodiment, the reference signal RAMP whose potential level gradually decreases with time is used.
First, in the period from time t0 to time t1, the control circuit 80 controls the control signal INI from the L level to the H level. Thereby, the p-channel transistor MPS is turned off. Further, in the same period from time t0 to time t1, the level of the reference signal RAMP is in a state of being higher than the level of the signal SIG, the gate potential of the p-channel transistor MP3 is a level close to the power supply voltage, and the p-channel transistor MP3 is in an off-state.
Accordingly, the potential of the node FBIN, which is the output node of the comparison circuit 34 and is also the input node of the positive feedback circuit 40, decreases to a potential close to the reference voltage (GND) due to current caused to flow by the current source 38. The potential of the node FBIN then decreases to the reference voltage, thereby the p-channel transistor MP6 is turned on, the n-channel transistor MN3 is turned off, and the potential of the node OUT, which is the output node of the positive feedback circuit 40, is at the H level close to the power supply voltage. Further, in response to the node OUT being at the H level, the p-channel transistor MP4 whose gate receives the potential of the node OUT is turned off. Further, in response to transition of the control signal INI from the H level to the L level at time t1, the p-channel transistor MPS is turned on.
At subsequent time t2, the reference signal generation circuit 48 starts changing the potential level of the reference signal RAMP. The reference signal generation circuit 48 outputs the reference signal RAMP having a ramp waveform whose potential gradually decreases with time, for example. Further, the counter circuit 54 starts count in synchronization with the start the change in the potential level of the reference signal RAMP and outputs a count signal indicating a count value to the count signal line 54a.
At subsequent time t3, it is assumed that the level of the reference signal RAMP decreases below the level of the signal SIG in accordance with the reset level of the pixel 12. In response, the gate potential of the p-channel transistor MP3 starts decreasing gradually from time t3, and thereby the potential of the node FBIN starts increasing gradually. Further, the potential of the gate of the p-channel transistor MP6 and the gate of the n-channel transistor MN3 connected to the node FBIN starts increasing gradually, and thereby the potential of the node OUT starts decreasing gradually.
Accordingly, at subsequent time t4, the p-channel transistor MP4 is turned on. In response, the node FBIN and the power supply voltage node are connected to each other, thereby positive feedback is applied to the potential of the node FBIN, and the potential of the node FBIN increases rapidly. As a result, the potential of the node OUT decreases rapidly.
In response to the rapid change in the potential of the node OUT, the pulse generator 46 generates and outputs one shot of pulse to the memory 52. The memory 52 holds a count value, which is indicated by a count signal being output from the counter circuit 54 at the timing of receiving the pulse from the pulse generator 46, as digital data of the signal SIG in accordance with the reset level of the pixel 12, that is, digital data of a noise signal.
At subsequent time t5, the reference signal generation circuit 48 resets the potential level of the reference signal RAMP to the reference level. Accordingly, the level of the reference signal RAMP returns to the state of being higher than the level of the signal SIG, the potential of the gate of the p-channel transistor MP3 reaches a level close to the power supply voltage, and the p-channel transistor MP3 is turned off. Further, in the period from time t5 to time t6, the control circuit 80 controls the control signal INI from the L level to the H level. Thereby, the p-channel transistor MP5 is turned off.
Accordingly, the potential of the node FBIN, which is the output node of the comparison circuit 34 and is also the input node of the positive feedback circuit 40, decreases to a potential close to the reference voltage due to current caused to flow by the current source 38. The potential of the node FBIN then decreases to the reference voltage, thereby the p-channel transistor MP6 is turned on, the n-channel transistor MN3 is turned off, and the potential of the node OUT, which is the output node of the positive feedback circuit 40, reaches the H level close to the power supply voltage. Further, in response to the node OUT reaching the H level, the p-channel transistor MP4 whose gate receives the potential of the node OUT is turned off. Further, in response to transition of the control signal INI from the H level to the L level at time t6, the p-channel transistor MPS is turned on.
At subsequent time t7, signal charge of the pixel 12 to be read out is transferred to the floating diffusion portion FD, thereby a signal (light detection signal) in accordance with the amount of signal charge as the signal SIG is output, and the level of the signal SIG of the output line 16 is reduced.
At subsequent time t8, the reference signal generation circuit 48 starts changing the potential level of the reference signal RAMP. Further, the counter circuit 54 newly starts count in synchronization with the start of the change in the potential level of the reference signal RAMP and outputs a count signal indicating a count value to the count signal line 54a.
At subsequent time t9, it is assumed that the level of the reference signal RAMP decreases below the level of the signal SIG in accordance with the amount of signal charge. In response, the gate potential of the p-channel transistor MP3 starts decreasing gradually from time t9, and thereby the potential of the node FBIN starts increasing gradually. Further, the potential of the gate of the p-channel transistor MP6 and the gate of the n-channel transistor MN3 connected to the node FBIN starts increasing gradually, and thereby the potential of the node OUT starts decreasing gradually.
Accordingly, at subsequent time t10, the p-channel transistor MP4 is turned on. In response, the node FBIN and the power supply voltage node are connected to each other, thereby positive feedback is applied to the potential of the node FBIN, and the potential of the node FBIN increases rapidly. As a result, the potential of the node OUT decreases rapidly.
In response to the rapid change in the potential of the node OUT, the pulse generator 46 generates and outputs one shot of pulse to the memory 52. The memory 52 holds a count value, which is indicated by a count signal being output from the counter circuit 54 at the timing of receiving the pulse from the pulse generator 46, as digital data of the signal SIG in accordance with the amount of signal charge, that is, digital data of a light detection signal.
The digital data of the noise signal and the digital data of the light detection signal held in the memory 52 in such a way are transferred to the output circuit 70 in accordance with a control signal from the horizontal scanning circuit 60. In the output circuit 70, the difference between a digital value of the noise signal and a digital value of the light detection signal is calculated by using a so-called digital CDS process and output to the external of the photoelectric conversion device 100 as a final AD conversion result.
In the comparator 32 of the present embodiment, with the application of the amplifier unit using the current source 38 in the output stage of the comparison circuit 34, it is possible to suppress a malfunction of the comparator 32. This feature will be described below.
It may be considered that, instead of the current source 38, a switch transistor to reset the potential of the node FBIN is provided to a portion in which the current source 38 is provided in the circuit illustrated in
During these periods, the switch transistor and the p-channel transistors MP3, MP4, and MPS are in the off-state. In this state, since off-leak current flowing when a transistor is in an off-state varies significantly, the potential of the node FBIN may undesirably start rising before the potential of the reference signal RAMP starts changing for a particular off-leak current of each transistor. In particular, such a phenomenon may occur in a comparator in which the sum of off-leak current flowing in the path of the p-channel transistors MP4 and MP5 and off-leak current flowing in the p-channel transistor MP3 is larger than the off-leak current of the switch transistor. As a result, if positive feedback is applied on the input signal and the potential of the node OUT is inverted before the potential of the reference signal RAMP starts changing, the memory 52 is unable to take in a count value in a period in which the potential of the reference signal RAMP changes (AD conversion period). In more detail, the memory 52 undesirably takes in the count initial value occurring before the start of count, and this will cause a large AD conversion error.
In this regard, in the comparator 32 of the present embodiment, since an amplifier unit using the current source 38 is applied on the output stage of the comparison circuit 34 and thereby a current path is provided between the node FBIN and the reference voltage node, the potential of the node FBIN during this period is stabilized at a level close to the reference voltage. Accordingly, it is possible to suppress a malfunction of the comparator 32 due to variation of the off-leak current flowing in the p-channel transistors MP3, MP4, and MP5 and reduce the AD conversion error.
As a load element of the amplifier unit 34b, the current source 38 configured as illustrated in
Further, it is possible to suppress a malfunction of the comparator 32 not only in the case where an active load such as the current source 38 is used but also in the case where a passive load such as the resistor R1 illustrated in
It is however preferable to use a current source load rather than a resistor as the load element of the amplifier unit 34b in terms of saving the area or in terms of suppressing interference between the positive feedback circuits 40 to reduce an AD conversion error.
When the resistor R1 illustrated in
Further, when the resistor R1 having a small resistance is used because a sufficient area is unable to be secured for the resistor R1 or the like, the current fluctuation on and after time t4 is larger, and there is a concern about an increase in the AD conversion error due to interference between positive feedback circuits. Although four comparison circuits 34 and four positive feedback circuits 40 are illustrated as a column circuit for four columns in
Further, in addition to a current fluctuation caused by the p-channel transistor MP4 being turned on, through-current flowing in the p-channel transistor MP6 and the n-channel transistor MN3 when the output signal of the positive feedback circuit 40 is inverted causes a potential fluctuation of the power supply voltage VDD or the reference voltage GND. The configuration of the current source 38 illustrated in
The circuit configuration of the comparator 32 illustrated in
In the configuration in which a switch transistor is provided instead of the current source 38 as described previously, the node FBIN is at the L level and in a floating state during a period in which the control signal INI is at the L level (time t1 to time t5 and on and after time t6). Since a floating state is a high impedance state, the potential of the node FBIN will easily fluctuate when a potential fluctuation of the power supply voltage VDD or the reference voltage GND as described previously occurs. Thus, interference due to inversion operations of other positive feedback circuits 40 may significantly increase the AD conversion error. Further, for some cases, when the node FBIN fluctuates, the transistor MP4 is temporarily turned on and positive feedback is applied to the input, and a malfunction may occur.
In this regard, in the circuit configuration of the comparator 32 illustrated in
Note that, in terms of reducing the impedance of the node FBIN, the bias voltage VB in
As described above, according to the present embodiment, it is possible to suppress a malfunction of the comparator 32 due to variation of off-leak current of transistors. It is therefore possible to reduce the AD conversion error by using such a comparator to form an AD conversion circuit. Further, it is possible to acquire a high quality image with a smaller AD conversion error by using such an AD conversion circuit to form a photoelectric conversion device.
Second EmbodimentA photoelectric conversion device according to a second embodiment of the present invention will be described with reference to
In the present embodiment, an example interconnection arrangement to a reference voltage pad connected to the comparator 32 will be described. Although description will be provided here with the circuit configuration of the first embodiment, the same can apply to the circuit configuration of other embodiments described later.
In
In the configuration example illustrated in
In the configuration example illustrated in
In the configuration example of
Note that, although both the source of the p-channel transistor MP5 and the source of the p-channel transistor MP6 are separated from the power supply voltage pad 92a and the interconnection 96a in the configuration example of
As described above, according to the present embodiment, it is possible to suppress influence on the comparison circuit 34 caused by a fluctuation of the power supply voltage VDD or the reference voltage GND due to an inversion operation in the positive feedback circuit 40 and further reduce the AD conversion error. Further, it is possible to suppress a malfunction of the comparator 32 due to variation of off-leak current of transistors in the same manner as in the first embodiment. It is therefore possible to reduce the AD conversion error by using such a comparator to form an AD conversion circuit. Further, it is possible to acquire a high quality image with a smaller AD conversion error by using such an AD conversion circuit to form a photoelectric conversion device.
Third EmbodimentA photoelectric conversion device according to a third embodiment of the present invention will be described with reference to
Although the configuration as the comparator 32 in which the signal SIG is input to the non-inverting input terminal of the comparison circuit 34 and the reference signal RAMP is input to the inverting input terminal of the comparison circuit 34 has been described in the previous embodiments, the configuration of the comparator 32 is not limited thereto. In the present embodiment, an example of a configuration in which the signal SIG is input to the inverting input terminal of the comparison circuit 34 and the reference signal RAMP is input to the non-inverting input terminal of the comparison circuit 34 will be described as a photoelectric conversion device including the comparator 32 having another configuration.
The drain of the n-channel transistor MN5 is connected to the input node of the positive feedback circuit 40 (node FBIN). The source of the n-channel transistor MN5 is connected to the drain of the n-channel transistor MN4. The source of the n-channel transistor MN4 is connected to the reference voltage node. The gate of the n-channel transistor MN4 is connected to the connection node between the drain of the p-channel transistor MP6 and the drain of the n-channel transistor MN3. The n-channel transistor MN5 is controlled by the control signal INI supplied from the control circuit 80.
In the period before time t0, the control signal INI is at the H level. Further, a signal in accordance with a reset level of the pixel 12 (noise signal) has been output as the signal SIG to the output line 16. Further, the reference level of the reference signal RAMP before the change in the potential level is started is higher than the level of the signal SIG in accordance with the reset level of the pixel 12.
First, in the period from time t0 to time t1, the control circuit 80 controls the control signal INI from the H level to the L level. Thereby, the n-channel transistor MN5 is turned off. Further, in the same period from time t0 to time t1, the level of the reference signal RAMP is in a state of being higher than the level of the signal SIG, the gate potential of the p-channel transistor MP3 is a level close to the power supply voltage, and the p-channel transistor MP3 is in an off-state. Accordingly, the potential of the node FBIN, which is the output node of the comparison circuit 34 and is also the input node of the positive feedback circuit 40, is at the H level.
At subsequent time t2, the reference signal generation circuit 48 starts changing the potential level of the reference signal RAMP. The reference signal generation circuit 48 outputs the reference signal RAMP having a ramp waveform whose potential gradually decreases with time, for example. Further, the counter circuit 54 starts count in synchronization with the start of the change in the potential level of the reference signal RAMP and outputs a count signal indicating a count value to the count signal line 54a.
At subsequent time t3, it is assumed that the level of the reference signal RAMP decreases below the level of the signal SIG in accordance with the reset level of the pixel 12. In response, the gate potential of the p-channel transistor MP3 starts increasing gradually from time t3, and thereby the potential of the node FBIN starts decreasing gradually. Further, the potential of the gate of the p-channel transistor MP6 and the gate of the n-channel transistor MN3 connected to the node FBIN starts increasing gradually, and thereby the potential of the node OUT starts increasing gradually.
Accordingly, at subsequent time t4, the n-channel transistor MN4 is turned on. In response, the n-channel transistor MM enters the on-state, the node FBIN and the reference voltage node are connected to each other, thereby positive feedback is applied to the potential of the node FBIN, and the potential of the node FBIN decreases rapidly. As a result, the potential of the node OUT increases rapidly.
In response to the rapid change in the potential of the node OUT, the pulse generator 46 generates and outputs one shot of pulse to the memory 52. The memory 52 holds a count value, which is indicated by a count signal being output from the counter circuit 54 at the timing of receiving the pulse from the pulse generator 46, as digital data of the signal SIG in accordance with the reset level of the pixel 12, that is, digital data of a noise signal.
At subsequent time t5, the reference signal generation circuit 48 resets the potential level of the reference signal RAMP to the reference level. Thereby, the level of the reference signal RAMP returns to the state of being higher than the level of the signal SIG, the potential of the gate of the p-channel transistor MP3 reaches a level close to the power supply voltage, and the p-channel transistor MP3 is turned off. Further, in the period from time t5 to time t6, the control circuit 80 controls the control signal INI from the L level to the H level. Thereby, the n-channel transistor MN5 is turned off. Accordingly, the potential of the node FBIN, which is the output node of the comparison circuit 34 and is also the input node of the positive feedback circuit 40, reaches the H level.
At subsequent time t7, signal charge of the pixel 12 to be read out is transferred to the floating diffusion portion FD, thereby a signal in accordance with the amount of signal charge (light detection signal) is output as the signal SIG, and the level of the signal SIG of the output line 16 is reduced.
At subsequent time t8, the reference signal generation circuit 48 starts changing the potential level of the reference signal RAMP. Further, the counter circuit 54 newly starts count in synchronization with the start of the change in the potential level of the reference signal RAMP and outputs a count signal indicating a count value to the count signal line 54a.
At subsequent time t9, it is assumed that the level of the reference signal RAMP decreases below the level of the signal SIG in accordance with the amount of signal charge. In response, the gate potential of the p-channel transistor MP3 starts increasing gradually from time t9, and thereby the potential of the node FBIN starts decreasing gradually. Further, the potential of the gate of the p-channel transistor MP6 and the gate of the n-channel transistor MN3 connected to the node FBIN starts increasing gradually, and thereby the potential of the node OUT starts increasing gradually.
Accordingly, at subsequent time t10, the n-channel transistor MN4 is turned on. In response, the n-channel transistor MN4 enters the on-state, the node FBIN and the reference voltage node are connected to each other, thereby positive feedback is applied to the potential of the node FBIN, and the potential of the node FBIN decreases rapidly. As a result, the potential of the node OUT increases rapidly.
In response to the rapid change in the potential of the node OUT, the pulse generator 46 generates and outputs one shot of pulse to the memory 52. The memory 52 holds a count value, which is indicated by a count signal being output from the counter circuit 54 at the timing of receiving the pulse from the pulse generator 46, as digital data of the signal SIG in accordance with the amount of signal charge, that is, digital data of a light detection signal.
Also in the comparator 32 of the present embodiment, as illustrated in
Note that, although both the source of the n-channel transistor MN5 and the source of the n-channel transistor MN3 are separated from the reference voltage pad 90a and the interconnection 94a in the configuration example of
Further, the reference voltage node arranged in the comparison circuit 34 and the reference node arranged in the positive feedback circuit 40 may be connected to the same reference voltage pad 90 via the separate interconnections 94a and 94b, respectively, in the same manner as in the configuration example illustrated in
As described above, according to the present embodiment, it is possible to suppress a malfunction of the comparator 32 due to variation of off-peak current of transistors. Further, in the same manner as in the first embodiment, it is possible to suppress a malfunction of the comparator 32 due to variation of off-peak current of transistors. It is therefore possible to reduce an AD conversion error by using such a comparator to form an AD conversion circuit. Further, it is possible to acquire a high quality image with a smaller AD conversion error by using such an AD conversion circuit to form a photoelectric conversion device.
Fourth EmbodimentA photoelectric conversion device according to a fourth embodiment of the present invention will be described with reference to
The photoelectric conversion device according to the present embodiment is the same as the photoelectric conversion device according to the third embodiment except for differences in the configuration of the comparator 32. That is, as illustrated in
In the comparator 32 of the present embodiment, as illustrated in
Also in the comparator 32 of the present embodiment, as illustrated in
Note that, although both the source of the n-channel transistor MN5 and the source of the n-channel transistor MN3 are separated from the reference voltage pad 90a and the interconnection 94a in the configuration example of
Further, the reference voltage node arranged in the comparison circuit 34 and the reference node arranged in the positive feedback circuit 40 may be connected to the same reference voltage pad 90 via the separate interconnections 94a and 94b, respectively, in the same manner as in the configuration example illustrated in
As described above, according to the present embodiment, it is possible to suppress a malfunction of the comparator 32 due to variation of off-peak current of transistors. Further, in the same manner as in the first embodiment, it is possible to suppress a malfunction of the comparator 32 due to variation of off-peak current of transistors. It is therefore possible to reduce an AD conversion error by using such a comparator to form an AD conversion circuit. Further, it is possible to acquire a high quality image with a smaller AD conversion error by using such an AD conversion circuit to form a photoelectric conversion device.
Note that, although the amplifier unit including the p-channel transistor MP7 and the current source 42 on the output stage of the comparison circuit 34 is provided in the present embodiment, a clip circuit that limits the level of the node FBIN may be provided.
Fifth EmbodimentA photoelectric conversion device according to a fifth embodiment of the present invention will be described with reference to
The photoelectric conversion device according to the present embodiment is the same as the photoelectric conversion device according to the first to fourth embodiments except for differences in the configuration of the comparator 32. That is, as illustrated in
When a positive feedback circuit 40 is formed using a logic gate such as a MOS inverter, through-current may flow from the power supply voltage node to the reference voltage node at transition of the input/output signal level. Such through-current typically has a full width at half maximum of around several nano seconds and a peak value of around several tens of micro amperes. Although
At this time, since a finite impedance is attached to the power supply voltage node and the reference voltage node, large through-current may cause a large potential fluctuation to occur in the power supply voltage VDD or the reference voltage GND. Accordingly, there will be a difference in the inversion timing in accordance with the number of positive feedback circuits 40 inverted at the same time, and a large error may occur in an AD conversion result. In other words, the AD conversion error may be significantly increased because of the interference due to inversion operations of other positive feedback circuits 40.
In this regards, in the present embodiment, with the use of an amplifier using a current source load (current source 44) for the positive feedback circuit 40, the potential fluctuation of the power supply voltage VDD or the reference voltage GND is suppressed. Although a reduction in the potential of the node FBIN at time t4 of
Also in the comparator 32 of the present embodiment, as illustrated in
Further, the reference voltage node arranged in the comparison circuit 34 and the reference node arranged in the positive feedback circuit 40 may be connected to the same reference voltage pad 90 via the separate interconnections 94a and 94b, respectively, in the same manner as in the configuration example illustrated in
In the comparator 32 of the present embodiment, with the use of the amplifier using the p-channel transistor MP8 and the current source 44, fluctuations of the power supply voltage VDD and the reference voltage GND due to an inversion operation of the positive feedback circuit 40 are suppressed. Therefore, as illustrated in
As described above, according to the present embodiment, it is possible to suppress a malfunction of the comparator 32 due to variation of off-peak current of transistors. It is therefore possible to reduce an AD conversion error by using such a comparator to form an AD conversion circuit. Further, it is possible to acquire a high quality image with a smaller AD conversion error by using such an AD conversion circuit to form a photoelectric conversion device.
Sixth EmbodimentA photoelectric conversion device according to a sixth embodiment of the present invention will be described with reference to
The photoelectric conversion device according to the present embodiment is the same as the photoelectric conversion device according to the first to fifth embodiments except for differences in the configuration of the comparator 32. That is, as illustrated in
The p-channel transistor MP9 is connected between the source of the p-channel transistor MP7 and the power supply voltage node. That is, the source of the p-channel transistor MP9 is connected to the power supply voltage node. The gate and the drain of the p-channel transistor MP9 are connected to the source of the p-channel transistor MP7. Further, the p-channel transistor MP10 is connected between the source of the p-channel transistor MP8 and the power supply voltage node. That is, the source of the p-channel transistor MP10 is connected to the power supply voltage node. The gate and the drain of the p-channel transistor MP10 are connected to the source of the p-channel transistor MP8.
With the p-channel transistor MP9 being provided between the p-channel transistor MP7 and the power supply voltage node, the gate-source voltage Vgs of the p-channel transistor MP7 drops by the gate-source voltage Vgs of the p-channel transistor MP9. Similarly, with the p-channel transistor MP10 being provided between the p-channel transistor MP8 and the power supply voltage node, the gate-source voltage Vgs of the p-channel transistor MP8 drops by the gate-source voltage Vgs of the p-channel transistor MP10. Accordingly, the off-leak current of the p-channel transistors MP7 and MP8 after the node FBIN transitions to the H level can be reduced, and the power consumption can be reduced. Further, a reduction of the off-leak current of the p-channel transistors MP7 and MP8 can suppress the relationship of current cancellation between the current sources 38 and the current source 42 from being compromised. As a result, it is possible to further suppress the potential fluctuation of the power supply voltage VDD or the reference voltage GND and reduce the AD conversion error.
As described above, according to the present embodiment, it is possible to suppress a malfunction of the comparator 32 due to variation of off-peak current of transistors. It is therefore possible to reduce an AD conversion error by using such a comparator to form an AD conversion circuit. Further, it is possible to acquire a high quality image with a smaller AD conversion error by using such an AD conversion circuit to form a photoelectric conversion device.
Seventh EmbodimentA photoelectric conversion device according to a seventh embodiment of the present invention will be described with reference to
That is, as illustrated in
Further, as illustrated in
With such a configuration of the comparison circuit 34, the comparison circuit 34 can have an offset cancellation function.
In the period before time t0, the control signals FB2 and FB1 are at the L level, and the control signal INI is at the H level. Further, a signal in accordance with the reset level of the pixel 12 (noise signal) is output to the output line 16 as the signal SIG.
First, at time t0, the control circuit 80 controls the control signal INI from the H level to the L level to turn off the n-channel transistor MN5. Further, at the same time t0, the control circuit 80 controls the control signals FB1 and FB2 from the L level to the H level to turn on the switches SW4, SW5, and SW6. Accordingly, if the p-channel transistors MP1, MP2, and MP3 all have the same size, current having a current value that is the same as the half the current value of the tail current source 36 flows in the p-channel transistor MP3. As a result, the gate-source voltage Vgs of the p-channel transistor MP3 in accordance with the current amount is held in the bias hold capacitor Cbh. Further, the potential that is a reference to the signal SIG and the reference signal RAMP is clamped in the clamp capacitors C1 and C2.
At subsequent time t1, the control circuit 80 controls the control signal FB2 from the H level to the L level. Thereby, the switch SW6 transitions from the on-state to the off-state.
At subsequent time t2, the control circuit 80 controls the control signal FB1 from the H level to the L level. Thereby, the switches SW4 and SW5 transition from the on-state to the off-state.
At subsequent time t3, the reference signal generation circuit 48 increases the potential of the reference signal RAMP by a predetermined voltage. Thereby, the potential of the node FBIN increases gradually. The potential of the node FBIN increases, and thereby, at time t4, the p-channel transistors MP7 and MP8 transition from the on-state to the off-state.
Next, at time t4 when the p-channel transistors MP7 and MP8 are turned off, the control circuit 80 controls the control signal INI to transition from the L level to the H level to turn on the n-channel transistor MN5.
At subsequent time t5, the reference signal generation circuit 48 starts changing the potential level of the reference signal RAMP. The reference signal generation circuit 48 outputs the reference signal RAMP having a ramp waveform whose potential gradually decreases with time, for example. Further, the counter circuit 54 starts count in synchronization with the start of the change in the potential level of the reference signal RAMP and outputs a count signal indicating a count value to the count signal line 54a.
At subsequent time t6, it is assumed that the level of the reference signal RAMP decreases below the level of the signal SIG in accordance with the reset level of the pixel 12. In response, the gate potential of the p-channel transistor MP3 starts increasing gradually from time t6, and thereby the potential of the node FBIN starts decreasing gradually.
In the present embodiment, because of the clamping operation, an inversion operation of the comparison circuit 34 starts from time t6 when the level of the reference signal RAMP becomes the same as that on and before time t3 regardless of variation of the thresholds of the n-channel transistors MN1, MN2, and the like (offset of the comparison circuit 34). That is, with the use of the comparator 32 of the present embodiment, it is possible to cancel influence due to the offset of the comparison circuit 34.
Since the operation on and after subsequent time t6 is the same as that of the fourth embodiment described with reference to
Note that, when the current of the tail current source 36 is the sub-threshold current of a transistor and is 10 nA, for example, the current flowing in the p-channel transistor MP3 in the period from time t0 to time t1 is around 5 nA. Further, the gate-source voltage Vgs of the n-channel transistor MN7 held in the bias hold capacitor Cbh becomes the potential below the threshold voltage in accordance with the current value of 5 nA. Herein, although the current of the tail current source 36 is sub-threshold current and is likely to vary, since a voltage in accordance with a current value including variation is held in the bias hold capacitor Cbh in the present embodiment, influence of variation is less likely to be notable. It is therefore possible to suppress variation of the inversion timings of the comparison circuits 34 affected by variation of current values of the tail current sources 36 or the like.
As described above, according to the present embodiment, it is possible to suppress a malfunction of the comparator 32 due to variation of off-peak current of transistors. It is therefore possible to reduce an AD conversion error by using such a comparator to form an AD conversion circuit. Further, it is possible to acquire a high quality image with a smaller AD conversion error by using such an AD conversion circuit to form a photoelectric conversion device.
Eighth EmbodimentA photoelectric conversion device according to an eighth embodiment of the present invention will be described with reference to
Although the photoelectric conversion device configured such that a single output line 16 is arranged on each column and pixel signals are read out on a row basis has been illustrated in the first to seventh embodiments, it is also possible to apply a configuration in which a plurality of output lines 16 are arranged on each column and pixel signals for a plurality of rows are simultaneously read out. In the present embodiment, a photoelectric conversion device in which a plurality of output lines 16 are arranged on each column and pixel signals for a plurality of rows can be simultaneously read out will be described.
In the photoelectric conversion device 100 according to the present embodiment, as illustrated in
In the configuration example illustrated in
The configuration example illustrated in
Note that, although
Further, although two output lines 16a and 16b are arranged on each column of the pixel array unit 10 in the configuration examples of
As described above, according to the present embodiment, it is possible to suppress a malfunction of the comparator 32 due to variation of off-peak current of transistors. It is therefore possible to reduce an AD conversion error by using such a comparator to form an AD conversion circuit. Further, it is possible to acquire a high quality image with a smaller AD conversion error by using such an AD conversion circuit to form a photoelectric conversion device.
Ninth EmbodimentA photoelectric conversion device according to a ninth embodiment of the present invention will be described with reference to
On each row of the pixel array unit 10, the control line 14 is arranged extending in the first direction (horizontal direction in
The output node of the pixel unit 22 corresponds to the node OUT of the positive feedback circuit 40 and is connected to the output line 16 on each column. The output line 16 on each column is connected to the pulse generator 46 on a corresponding column. The circuit configuration of the pulse generator 46 and on the downstream stage thereof is the same as that in the previous embodiments.
The control signal INT is supplied to the plurality of pixel units 22 arranged in the pixel array unit 10 from the control circuit 80 via the control line 80a. Further, the reference signal RAMP is supplied to the plurality of pixel units 22 arranged in the pixel array unit 10 from the reference signal generation circuit 48 via the reference signal line 48a.
The pixel 12 may be formed of a photoelectric converter PD, a transfer transistor M1, a reset transistor M2, and an overflow transistor M5, as illustrated in
The photoelectric converter PD is a photodiode, for example, the anode thereof is connected to a reference voltage node, and the cathode thereof is connected to the source of the transfer transistor M1 and the source of the overflow transistor M5. The drain of the transfer transistor M1 is connected to the source of the reset transistor M2. The connection node of the drain of the transfer transistor M1 and the source of the reset transistor M2 is a floating diffusion portion FD. The floating diffusion portion FD corresponds to the output node of the pixel 12 and is connected to the gate of the n-channel transistor MN2 that is a non-inverting input terminal of the comparison circuit 34. The drain of the reset transistor M2 is connected to the connection node of the drain of the p-channel transistor MP2, the drain of the n-channel transistor MN2, and the gate of the p-channel transistor MP3. The drain of the overflow transistor M5 is connected to the power supply voltage node.
In the case of the circuit configuration of
A control signal ΦTX that is a drive pulse used for controlling the transfer transistor M1 is supplied to the signal line TX from the vertical scanning circuit 20. A control signal ΦRES that is a drive pulse used for controlling the reset transistor M2 is supplied to the signal line RES from the vertical scanning circuit 20. A control signal ΦOFG that is a drive pulse used for controlling the overflow transistor M5 is supplied to the signal line OFG from the vertical scanning circuit 20. When each transistor is formed of an n-channel transistor, a corresponding transistor is turned on in response to a control signal at the H level being supplied from the vertical scanning circuit 20. Further, a corresponding transistor is turned off in response to a control signal at the L level being supplied from the vertical scanning circuit 20.
Note that, although
In the period before time t1, the control signals ΦOFG, ΦRES, ΦTX, and INI are at the L level. Further, the reference signal RAMP is at a predetermined reference level.
In the period from time t1 to time t2, the vertical scanning circuit 20 controls the control signal ΦOFG of a row to be read out from the L level to the H level. Thereby, the overflow transistors M5 of the pixel units 22 belonging to a corresponding row are turned on, and charge of the photoelectric converters PD is reset. The timing when the control signal ΦOFG transitions from the H level to the L level is a start timing of an exposure period in the photoelectric converter PD.
In the subsequent period from time t3 to time t4, the vertical scanning circuit 20 controls the control signal ORES of a row to be read out from the L level to the H level. Thereby, the reset transistors M2 of the pixel units 22 belonging to a corresponding row are turned on, and charge of the floating diffusion portion FD is reset. The timing when the control signal ΦOFG transitions from the H level to the L level is a start timing of an exposure period in the photoelectric converter PD.
Further, in the period from time t3 to time t5, the control circuit 80 controls the control signal INI from the L level to the H level. Thereby, the p-channel transistor MP5 is turned off. Further, in the same period from time t0 to time t1, the level of the reference signal RAMP is in a state of being higher than the level of the signal SIG, the gate potential of the p-channel transistor MP3 is a level close to the power supply voltage, and the p-channel transistor MP3 is in an off-state. Accordingly, the potential of the node FBIN, which is the output node of the comparison circuit 34 and is also the input node of the positive feedback circuit 40 decreases to a potential close to the reference voltage due to the current caused to flow by the current source 38.
In the subsequent period from time t6 to time t7, the reference signal generation circuit 48 changes the potential level of the reference signal RAMP gradually and performs AD conversion on a signal in accordance with the reset level of the pixel 12.
In the subsequent period from time t8 to time t9, the vertical scanning circuit 20 controls the control signal ΦTX of a row to be read out from the L level to the H level. The timing when the control signal ΦTX transitions from the H level to the L level is an end timing of the exposure period in the photoelectric converter PD. Thereby, charge generated and accumulated in the photoelectric converter PD during the exposure period is transferred to the floating diffusion portion FD. The potential of the floating diffusion portion FD, which is also the inverting input terminal of the comparison circuit 34, is reduced to a potential in accordance with the amount of charge transferred from the photoelectric converter PD.
Further, in the same period from time t8 to time t9, the control circuit 80 controls the control signal INI from the L level to the H level. Thereby, the p-channel transistor MP5 is turned off. Further, in the same period from time t8 to time t9, the level of the reference signal RAMP is in a state of being higher than the level of the signal SIG, the gate potential of the p-channel transistor MP3 is a level close to the power supply voltage, and the p-channel transistor MP3 is in an off-state. Accordingly, the potential of the node FBIN, which is the output node of the comparison circuit 34 and is also the input node of the positive feedback circuit 40 decreases to a potential close to the reference voltage due to the current caused to flow by the current source 38.
In the subsequent period from time t10 to time t11, the reference signal generation circuit 48 changes the potential level of the reference signal RAMP gradually and performs AD conversion on a signal in accordance with the amount of signal charge.
Note that, although
As described above, according to the present embodiment, it is possible to suppress a malfunction of a comparator due to variation of off-peak current of transistors. It is therefore possible to reduce an AD conversion error by using such a comparator to form an AD conversion circuit. Further, it is possible to acquire a high quality image with a smaller AD conversion error by using such an AD conversion circuit to form a photoelectric conversion device.
Note that, although the pixel unit 22 is formed of the pixel 12, the comparison circuit 34, and the positive feedback circuit 40 in the present embodiment, the pixel unit 22 may further include the pulse generator 46 or further include the pulse generator 46 and the memory 52.
Tenth EmbodimentA photoelectric conversion device according to a tenth embodiment of the present invention will be described with reference to
The photoelectric conversion device described in the first to ninth embodiments may be configured such that all the circuit blocks are arranged on a single substrate or such that a plurality of substrates are stacked to form a stacked type device and the circuit blocks are divided into respective substrates.
Note that circuit elements forming one function block are not necessarily required to be arranged on the same substrate and may be arranged on respective substrates. For example, out of the circuit elements forming the comparison circuit 34, the p-channel transistors MP1 and MP2 forming a current mirror may be arranged on one circuit substrate, and the n-channel transistors MN1 and MN2 forming a differential pair may be arranged on another circuit substrate. Alternatively, the p-channel transistors MP1 and MP2 forming a current mirror may be arranged on a circuit substrate, and the re-channel transistors MN1 and MN2 forming a differential pair may be arranged on a pixel substrate.
Eleventh EmbodimentAn imaging system according to an eleventh embodiment of the present invention will be described with reference to
The photoelectric conversion device 100 described in the above first to tenth embodiments can be applied to various imaging systems. Examples of applicable imaging systems may include a digital still camera, a digital camcorder, a surveillance camera, a copying machine, a fax machine, a mobile phone, an on-vehicle camera, an observation satellite, and the like. In addition, a camera module including an optical system such as a lens and an imaging device is also included in the imaging system.
An imaging system 200 illustrated as an example in
Further, the imaging system 200 includes a signal processing unit 208 that processes an output signal output from the imaging device 201. The signal processing unit 208 generates image data from digital signals output by the imaging device 201. Further, the signal processing unit 208 performs operations of performing various correction or compression to output image data, if necessary.
Furthermore, the imaging system 200 includes a memory unit 210 for temporarily storing image data therein and an external interface unit (external I/F unit) 212 for communicating with an external computer or the like. The imaging system 200 further includes a storage medium 214 such as a semiconductor memory for performing storage or readout of imaging data and a storage medium control interface unit (storage medium control I/F unit) 216 for performing storage or readout on the storage medium 214. Note that the storage medium 214 may be embedded in the imaging system 200 or may be removable.
Furthermore, the imaging system 200 includes a general control/operation unit 218 that performs various calculation and controls the entire digital still camera and a timing generation unit 220 that outputs various timing signals to the imaging device 201 and the signal processing unit 208. Here, the timing signal or the like may be input from the outside, and the imaging system 200 may include at least the imaging device 201 and the signal processing unit 208 that processes an output signal output from the imaging device 201.
The imaging device 201 outputs an imaging signal to the signal processing unit 208. The signal processing unit 208 performs predetermined signal processing on an imaging signal output from the imaging device 201 and outputs image data. The signal processing unit 208 uses an imaging signal to generate an image.
As described above, according to the present embodiment, the imaging system to which the photoelectric conversion device 100 according to any of the first to tenth embodiments is applied can be realized.
Twelfth EmbodimentAn imaging system and a movable object according to a twelfth embodiment of the present invention will be described with reference to
The imaging system 300 is connected to the vehicle information acquisition device 320 and can acquire vehicle information such as a vehicle speed, a yaw rate, a steering angle, or the like. Further, the imaging system 300 is connected to a control ECU 330, which is a control device that outputs a control signal for causing a vehicle to generate braking force based on a determination result by the collision determination unit 318. Further, the imaging system 300 is also connected to an alert device 340 that issues an alert to the driver based on a determination result by the collision determination unit 318. For example, when the collision probability is high as the determination result of the collision determination unit 318, the control ECU 330 performs vehicle control to avoid a collision or reduce damage by applying a brake, pushing back an accelerator, suppressing engine power, or the like. The alert device 340 alerts a user by sounding an alert such as a sound, displaying alert information on a display of a car navigation system or the like, providing vibration to a seat belt or a steering wheel, or the like.
In the present embodiment, an area around a vehicle, for example, a front area or a rear area is captured by using the imaging system 300.
Although the example of control for avoiding a collision to another vehicle has been described above, the embodiment is applicable to automatic driving control for following another vehicle, automatic driving control for not going out of a traffic lane, or the like. Furthermore, the imaging system is not limited to a vehicle such as the subject vehicle and can be applied to a movable object (moving apparatus) such as a ship, an airplane, or an industrial robot, for example. In addition, the imaging system can be widely applied to a device which utilizes object recognition, such as an intelligent transportation system (ITS), without being limited to movable objects.
Modified EmbodimentsThe present invention is not limited to the embodiments described above, and various modifications are possible.
For example, an example in which a part of the configuration of any of the embodiments is added to another embodiment or an example in which a part of the configuration of any of the embodiments is replaced with a part of the configuration of another embodiment is also one of the embodiments of the present invention.
Further, the pixel circuit illustrated in
Further, although the examples in which the present invention is applied to the photoelectric conversion device in the first to tenth embodiments have been illustrated, a comparator, an AD converter, or the like to which the present invention is applied can be realized.
Further, the photoelectric conversion device described in each of the above first to tenth embodiments can be configured as a device intended for acquisition of an image, that is, an imaging device. Alternatively, in a case of application to a device mainly intended for ranging as described in the above fourteenth embodiment, it is not necessarily required to output an image. In such a case, it can be said that such a device is a photoelectric conversion device that converts optical information into a predetermined electrical signal. The imaging device is one of the photoelectric conversion devices.
Further, the imaging systems illustrated in the above eleventh and twelfth embodiments are examples of an imaging system to which the photoelectric conversion device of the present invention may be applied, and an imaging system to which the photoelectric conversion device of the present invention can be applied is not limited to the configuration illustrated in
While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.
This application claims the benefit of Japanese Patent Application No. 2020-030610, filed Feb. 26, 2020, which is hereby incorporated by reference herein in its entirety.
Claims
1. A comparator comprising:
- a comparison circuit including a differential unit that compares an input signal with a reference signal and changes a level of a signal output to a first node in accordance with a result of comparison and an amplifier unit that includes a load element and outputs a signal in accordance with a potential of the first node to a second node; and
- a positive feedback circuit that is connected to the second node and a third node and changes a level of a signal at the third node at a rate higher than a change rate of a level of a signal at the second node in accordance with a change in a level of a signal at the second node.
2. The comparator according to claim 1, wherein the amplifier unit of the comparison circuit includes a first transistor of a first conductivity type whose first main node is connected to a first voltage node supplied with a first voltage and whose control node is connected to the first node and a first current source load connected between a second voltage node supplied with a second voltage, which is different from the first voltage, and a second main node of the first transistor.
3. The comparator according to claim 2, wherein the first current source load includes a second transistor that forms a gate grounded circuit and whose control node is supplied with a bias voltage that is different from a reference voltage.
4. The comparator according to claim 3, wherein the first current source load further includes a bias hold capacitor connected to the control node of the second transistor.
5. The comparator according to claim 1, wherein the amplifier unit of the comparison circuit includes a first transistor of a first conductivity type whose first main node is connected to a first voltage node supplied with a first voltage and whose control node is connected to the first node and a resistor connected between a second voltage node supplied with a second voltage, which is different from the first voltage, and a second main node of the first transistor.
6. The comparator according to claim 2, wherein the amplifier unit of the comparison circuit further includes a third transistor of the first conductivity type whose first main node is connected to the first voltage node and whose control node is connected to the second node and a second current source load connected between a second main node of the third transistor and the second voltage node.
7. The comparator according to claim 6, wherein the amplifier unit of the comparison circuit further includes a fourth transistor of the first conductivity type connected between the first main node of the third transistor and the first voltage node.
8. The comparator according to claim 2, wherein the positive feedback circuit includes an inverter that inverts a level of a signal at the second node and outputs the inverted level of the signal to the third node and a feedback unit that positively feeds back a signal in accordance with a potential of the third node to the second node.
9. The comparator according to claim 8, wherein the third node is an output node of the positive feedback circuit.
10. The comparator according to claim 2, wherein the positive feedback circuit includes an amplifier unit that outputs a signal in accordance with a potential of the second node to the third node and a feedback unit that positively feeds back a signal in accordance with a potential of the third node to the second node.
11. The comparator according to claim 10, wherein the second node is an output node of the positive feedback circuit.
12. The comparator according to claim 10, wherein the amplifier unit of the positive feedback circuit further includes a fifth transistor of a first conductivity type whose first main node is connected to a third voltage node supplied with the first voltage and whose control node is connected to the second node and a third current source load connected between a fourth voltage node supplied with the second voltage and a second main node of the fifth transistor.
13. The comparator according to claim 12, wherein the amplifier unit of the positive feedback circuit further includes a sixth transistor of the first conductivity type connected between the first main node of the fifth transistor and the third voltage node.
14. The comparator according to claim 8, wherein the feedback unit further includes
- a seventh transistor of a first conductivity type whose first main node is connected to a third voltage node supplied with the first voltage, and
- an eighth transistor of the first conductivity type whose first main node is connected to a second main node of the seventh transistor, whose second main node is connected to the second node, and whose control node is connected to the third node.
15. The comparator according to claim 14 further comprising:
- a pad electrode used for supplying the first voltage;
- a first interconnection connected between the pad electrode and the first voltage node; and
- a second interconnection connected between the pad electrode and the third voltage node.
16. The comparator according to claim 14 further comprising:
- a first pad electrode and a second pad electrode used for supplying the first voltage;
- a first interconnection connected between the first pad electrode and the first voltage node; and
- a second interconnection connected between the second pad electrode and the third voltage node.
17. The comparator according to claim 8, wherein the feedback unit further includes
- a ninth transistor of a second conductivity type whose first main node is connected to the second node, and
- a tenth transistor of the second conductivity type whose first main node is connected to a second main node of the ninth transistor, whose second main node is connected to a fourth voltage node supplied with the second voltage, and whose control node is connected to the third node.
18. The comparator according to claim 17 further comprising:
- a pad electrode used for supplying the second voltage;
- a third interconnection connected between the pad electrode and the second voltage node; and
- a fourth interconnection connected between the pad electrode and the fourth voltage node.
19. The comparator according to claim 17 further comprising:
- a third pad electrode and a fourth pad electrode used for supplying the second voltage;
- a third interconnection connected between the third pad electrode and the fourth voltage node; and
- a fourth interconnection connected between the fourth pad electrode and the second voltage node.
20. The comparator according to claim 2, wherein the first voltage is a power supply voltage, and the second voltage is a reference voltage.
21. The comparator according to claim 1, wherein the comparison circuit includes a clamp capacitor provided to a node to which the input signal is input and a clamp capacitor provided to a node to which the reference signal is input.
22. An analog-to-digital converter comprising:
- the comparator according to claim 1; and
- a counter circuit that outputs, as digital data of the input signal, a count value in accordance with a length of a period from start of comparison between the input signal and the reference signal to a change of a level of the signal output to the first node.
23. A photoelectric conversion device comprising:
- a plurality of pixels arranged so as to form a plurality of rows and a plurality of columns and each including a photoelectric converter;
- a plurality of output lines each arranged on each of the plurality of columns and each connected to pixels on a corresponding column; and
- a plurality of analog-to-digital converters according to claim 22 each connected to each of the plurality of output lines and configured to perform analog-to-digital conversion on pixel signals output from pixels on a corresponding column.
24. The photoelectric conversion device according to claim 23, wherein two or more of the output lines are arranged on each of the plurality of columns
25. The photoelectric conversion device according to claim 24, wherein comparison circuits and positive feedback circuits of two or more of the analog-to-digital converters arranged on the same column are arranged close to each other for each of the comparison circuits and the positive feedback circuits.
26. A photoelectric conversion device comprising:
- a plurality of pixel units arranged so as to form a plurality of rows and a plurality of columns and each including a pixel including a photoelectric converter and the comparator according to claim 1 in which a signal in accordance with charge generated in the photoelectric converter of the pixel is the input signal;
- a plurality of output lines arranged on each of the plurality of columns and each connected to pixel units on a corresponding column; and
- a counter circuit that is provided in association with each of the plurality of output lines and outputs, as digital data of the input signal, a count value in accordance with a length of a period from start of comparison between the input signal and the reference signal to a change of a level of a signal output from pixel unit on a corresponding column.
27. The photoelectric conversion device according to claim 23 comprising:
- a first substrate on which the photoelectric converter is provided; and
- a second substrate that is stacked on the first substrate and on which at least a part of the comparator is provided.
28. An imaging system comprising:
- the photoelectric conversion device according to claim 23; and
- a signal processing unit that processes a signal output from the photoelectric conversion device.
29. A movable object comprising:
- the photoelectric conversion device according to claim 23;
- a distance information acquisition unit that acquires distance information on a distance to an object, from a parallax image based on signals from the photoelectric conversion device; and
- a control unit that controls the movable object based on the distance information.
Type: Application
Filed: Feb 3, 2021
Publication Date: Aug 26, 2021
Inventors: Hideo Kobayashi (Tokyo), So Hasegawa (Kanagawa)
Application Number: 17/166,346
