Focal plane processor for IR detection
A focal plane array detector in a sensor for IR imaging. Pixel readout and analog to digital conversion is performed in an integrated circuit which includes the imaging array. The charge of each pixel is accumulated in a capacitor and a digital to analog charge converter cancels this charge. A controller controls this charge converter providing the appropriate amount of charge. An analog to digital converter incorporating a comparator switches the controller on/off. In some embodiment a programmable device external to the sensor participates in the control of the analog to digital converter. In a typical embodiment of the invention a cycle generator controls the charge output of the digital to analog converter. In a preferred embodiment the analog to digital converters are grouped wherein all such converters group operate simultaneously.
The present invention relates generally to IR focal plane array detectors. More specifically, the present invention is in the field of detector pixel signal readout and processing.
BACKGROUND OF THE INVENTIONPhotonic detector devices respond to received photons by creating an electric effect which can be quantified and hence provide information as regards the flux of the received photons. Focal plane arrays (FPAs) of detectors are used to obtain images of objects, whereby each detector provides a pixel in the image array. In the image, each pixel is provided with a unique address and a numeric value, which can further be used for manipulating the image for extracting information from the image.
Thermal Infra red (IR) FPAs are used for obtaining images in the thermal IR range. The resolution of an imaging array in the IR is limited by two factors, namely noise equivalent temperature (NET) and spatial noise, of the analog chain: the focal plane processor (FPP), the external analog circuitry and the analog to digital conversion. An external interference, such as RF interference or signals induced by the digital system may also degrade the performance. The temporal noise of the FPP can be reduced by careful design to be negligible relative to the signal noise, providing low NET values. Yet, to overcome the spatial noise, an external process of non uniformity correction (NUC) must be carried out. A “two point correction” process is generally used, assuming the analog chain to be linear and having stable offset and gain. The final results depend upon the validity of these assumptions, and the spatial resolution becomes a main restriction, limiting the dynamic range and requiring frequent calibrations. The FPP is kept at a steady temperature, e.g. at about 77° K while the external circuitry is exposed to wide temperature variations, degrading the gain and offset stability. Also, the FPP is relatively shielded against external interference, which is not the case with the external elements. In order to reach such a low temperature and keep it steady, the sensor is disposed within a cooled dewar. The dewar has electric connections installed in the walls, in order to facilitate electrical connection with external circuits.
Heat production is a matter of high importance in the design of FPPs. The efficiency of the cooling system being very low, the amount of power consumed by the circuits of the FPP at any time must also be limited. Sampling rates, sampling resolution, and output rates are factors which affect heat production. The use of components and combinations thereof having a low power consumption is favoured in the design of cooled FPPs. A trade-off between imaging parameters is a compromise often imposed for obtaining desirable image parameters. Accordingly, image frame rate at the expense of image resolution or image size are known in the art. Continuous attempts are made by various system designers to overcome the limitations described, striving to attain a high frame rate and a high resolution, with a reasonable power dissipation.
The value of the electric effect associated with each detector of the FPA is converted into a digital format, before it can be subjected to digital image processing. However, there are a number of critical reasons favouring conversion to digital format at an early stage as possible of the signal corresponding to each pixel. U.S. Pat. No. 5,886,659 discloses an on-focal plane analog to digital conversion application. Benefits of having the analog-to-digital conversion in the focal plane are discussed in that publication, the contents of which are incorporated herewith by reference.
Integrated converters have used several types of conversion schemes. Single-slope analog to digital conversion is a well-established technique. For example, U.S. Pat. No. 5,742,058 discloses an analog to digital conversion in the focal plane utilizing a single slope conversion cycle. The Dual-slope conversion provides better accuracy and stability (R. Van De Plassche, cited below). Yet, both techniques require quite long conversion times, being based on counters: the full-scale time of an n-bit single-slope converter is 2n*Tc, where Tc is the clock cycle time. The dual slope requires up to twice that time. The dual-ramp single-slope method, described by R. Van De Plassche in: INTEGRATED ANALOG TO DIGITAL AND DIGITAL TO ANALOG CONVERTERS, Kluwer Academic Press, pp. 274-289, 1994, the contents of which are incorporated herein by reference, promises both high resolution and reduced conversion time.
It is on the background hitherto described, that the present invention was conceived. The invention described below provides a basis for increasing the rate of sampling digitally at the FPP level, without compromising the resolution of each pixel.
BRIEF DESCRIPTION OF THE DRAWINGS
A system of the invention is implemented in association with an integrated circuit device, a focal plane processor (FPP). The FPP is disposed within a cooled dewar, as described schematically in
The process of the invention is summarized schematically in
The charge accumulated in the pixel in the FPP of the invention is according to the present invention converted to a digital form by a circuit modified from Van De Plassche, cited above. The original circuit, known as a dual ramp single slope converter, converts input voltage into a digital number. The modification of the circuit to a charge converter circuit, and the function thereof are described in reference to
However, in practice, the pixels are arranged in the framework of a matrix of rows and columns which may be long such that certain implications are inevitable trying to adapt a solitary pixel ADC application to a pixel array application, such as a column of the array . Accordingly, the following two concerns must be dealt with. First, the long column line adds a parasitic capacitance, Ccl, the value of which is about one order of magnitude larger then the pixel capacitor Cpx. Second, during the conversion, a cross-talk exists between adjacent column lines, impairing the function of the matrix.
In accordance with the present invention, an integrator, the input of which is a virtual ground, is added as a first stage of the comparator of the charge ADC circuit. This architectural feature helps to overcome the above mentioned problems as the column line in this configuration becomes connected to a virtual ground. Such a configuration is described in
The use of an FPA of the invention is typically in IR imaging applications, in which accurate reference to the absolute temperature of a pixel is not required. Rather, in an IR imaging application, the accuracy of the differences among the different pixels is the matter of relevance. Accurate variability measurement and conversion can be supported by the “dual ramp single slope ADC” used in some embodiments of the present invention.
The “dual ramp single slope ADC” employs a quantification cycle which implements two steps. In the first step MS (most significant) part of the numeric value is determined, and in a second step, LS (least significant) part of the numeric value of the charge is determined. The two parts of the digital values of the measured quantity must be combined to form a complete digital word, as will be explained later on. In the framework of a pixel array, the readout and quantification are performed in the context of columns and rows as has been discussed above, and will be further explained with reference to
At time t0, the quiescent output voltage of integrator 90, is forced by closing momentarily switch S1 91 to Vpch, (pre-charge voltage), which is also the reference voltage for comparator 92. Closing momentarily switch S3 94 at time t1, the charge of a pixel, accumulated in capacitor Cpxl 96, is transferred to integrator 90, lowering the output voltage accordingly. Closing switch S4 98 at time t2, a constant current source is established by the combination of a linear ramp voltage (ramp 148 of
The output of the integrator goes down and reaches Vpch at time t6, disabling the comparator. The counter reading is sampled at time t6, providing the LS count to be subtracted (with the proper weight ) from the digital number represented by the former MS bits. The ratio between the absolute values of the slopes of the two ramps determines the number and weight of LS bits. The value of Δt is nominally less then one clock time. Yet, in practice, due to delays in the comparator and the switching, it might reach as high as some clock times, the actual number of which depends also upon the clock frequency. Consequently, the period and the number of counter stages assigned to the second ramp is to be increased accordingly.
Being used in IR imagers, an offset thus created is compensated for in the non-uniformity correction (NUC) procedure, provided it has a constant value. An advantage associated with the ADC implementation in accordance with the present invention, results from the comparator being activated at a constant input level by a signal of a constant slope, independent of the converted signal amplitude. Also, the focal plane is cooled to a very stable temperature. Therefore, the delays and the resulting offset are very stable. Another advantage obtained by using the invented ADC in focal plane arrays is the connection of the column line to a virtual ground. The column lines are long and parallel, resulting in a significant coupling capacitance between neighbors. During the period of transfer of charge from the pixel to the ADC, the column line potential is temporarily raised, only to revert to the original value by the end of that period. Therefore, finally no charge injection occurs between adjacent column lines.
The combination a common linear voltage ramp (148 or 150 in
In order to further decrease the conversion time without loosing resolution, the “dual ramp single slope ADC” concept can be further extended into a multi ramp ADC, where additional conversion steps, having successively decreased weights, are employed. This may be desirable in the case of very high resolution converters.
Programmable Logic Applications in the Present InventionThe FPP of the present invention is applicable to IR imagers in general. Yet, it is especially advantageous in cooled imagers, where the heat dissipated in the focal plane should be removed by cooling to a low and stable temperature. Therefore, architectural and functional considerations of the make-up of the integrated circuit have been made in order to decrease the power dissipation in the FPP integrated circuit. In accordance with one aspect of the invention, a programmable logic device, external to the dewar in which the integrated circuit of the sensor is disposed, performs several tasks. The first one relates to the digital functions of the ADCs which are partially performed outside of the dewar. These functions being performed in the uncooled environment, serve to decrease the amount of components and heat dissipation in the FPP, therefore potentially improving the overall performance of the imager. As explained above, the digital value of each pixel is obtained in a MS part and an LS part. The two parts, after initial processing in the FPP, are finally combined to form one digital word in the external programmable logic device. The second task relates to the remapping of the pixels. The data transmission from the FPP to the programmable logic is optimized to minimize focal plane power dissipation. Consequently, the bits of a digital word transmitted from the dewar include a mixture of data from two pixels. Such mixed data is remapped in the external logic device, to restore the image.
The external programmable logic device also adds flexibility and optimizes the conversion cycle. Whereas the order of succession of the various conversion steps is fixed, the duration of each step is controlled externally. Thus, the duration of each step is set as required by the actual performance sequence, avoiding the need to reserve lengthy intervals of time for confidence margins. The data transfer associated with this aspect of the invention is described with reference to
Through the conventional comparatively slow communication bus, various operation parameters of the FPP, which are not directly involved in the timing of the analog-to-digital conversion steps, are controlled, These include, inter alia, “integrate-then-read” (ITR) or “integrate-while-read” (IWR) modes of operation, working points, readout window size and location, etc. One parameter should be mentioned here, namely the resolution of the analog-to-digital conversion. The duration of a conversion cycle depends also on the resolution, increasing with the resolution. Some bits of the slow communication stream control the resolution, e.g. a 13 bit or a 15 bit resolution, enabling a trade-off between the resolution and the frame-rate.
A faster link carries to the FPP the clock signal and the timing information of the conversion steps, mentioned earlier. In this fast channel, a stream of short pulses is sent on a wire from the external programmable logic to the FPP, where each pulse is recognized in the FPP as the start or end of a specific step. Programming the intervals between the pulses, the optimal conversion duration is determined. The raw data bus carries the partially processed data produced by the ADCs, from the FPP to be finally processed and remapped in the programmable logic device.
A different aspect of the imaging array which relates to the external programmable logic, is its exploitation as a user interface. Thus, in some embodiments of the invention, the programmable logic device can be used also as an interface to the external system 190 in
As described so far, the system of the invention is most beneficial in cooled FPPs, typically implemented in thermal IR imaging. It is nevertheless contended that the same system can be used in any FPP systems, for other imaging purposes, such as visible, near infra-red and X-ray. In such systems, heat dissipation in the FPP may be less critical, but the benefits of the system are still evident.
Claims
1. A focal plane array containing rows and columns of pixels for IR imaging, wherein pixel readout and analog to digital conversion is performed in an integrated circuit inclusive of said array, comprising:
- at least one capacitor per pixel, for accumulating the charge of a detector element.
- at least one analog to digital converter (ADC) incorporating a comparator for on/off switching a digital controller, wherein said controller controls a digital to analog charge converter for providing an appropriate amount of charge to cancel out the charge of one pixel at a time by one ADC, and
- switching elements for connecting said pixels to said ADCs.
2. A focal plane array for IR imaging as in claim 1 comprising a cycle generator, for controlling a charge output of a digital to analog converter to cancel out said charge of said pixel.
3. A focal plane array for IR imaging as in claim 2 wherein said cycle generator is a ramp generator connected through a capacitor to the input port of said at least one ADC and to a column line.
4. A focal plane array for IR imaging as in claim 3 wherein said ramp generator produces at least one linear ramp.
5. A focal plane array for IR imaging as in claim 1 wherein said comparator includes an integrator as an input stage.
6. A focal plane array for IR imaging as in claim 1 and wherein said focal plane array is cooled.
7. A focal plane array for IR imaging as in claim 1 wherein said ADCs are allocated in groups, wherein all ADCs of one group operate simultaneously.
8. A focal plane array for IR imaging as in claim 7 each of said groups contain at least two ADCs per each column.
9. A focal plane array for IR imaging as in claim 7 and wherein one multi ramp generator serves at least one group of ADCs.
10. A focal plane array for IR imaging as in claim 7 and wherein one counter serves at least one group of ADC's.
11. An IR imaging system as in claim 1 and wherein an external programmable logic device connected via a communication channel participates at least in the control of said ADC.
12. An IR imaging system as in claim 11 wherein said external programmable device also contains a buffer, for providing a interface between the pixels' readout stream and the timing parameters and communications protocols of a user.
13. A method for digitally quantifying IR radiation impinging on a focal plane array, wherein pixels readout and analog to digital conversion is performed in an integrated circuit inclusive of said array, employing at least one circuit for each column of said array, and wherein a charge on each of said pixels is converted to a digital number, comprising the steps of:
- on/off switching by a comparator of a digital controller controlling a digital to analog charge converter for supplying a charge by a digital to analog converter in an appropriate quantity to cancel out said charge of said pixel, and
- generating a dual ramp conversion cycle for a substantially two step analog to digital conversion of said charge and providing a most significant count in a first step and a least significant count of said charge in a second step.
14. A method for digitally quantifying IR radiation impinging on a focal plane array as in claim 13 wherein an analog to digital conversion of the charge in every pixel is done on the focal plane in two quantification steps, a first step providing the most significant bits and a second step the least significant bits of said quantification.
15. A method for digitally quantifying IR radiation impinging on a focal plane array as in claim 14 and wherein the combination of the most significant bits and the least significant bits into one digital number is done partially externally, outside said focal plane array.
16. A method for digitally quantifying IR radiation impinging on a focal plane array as in claim 13 and wherein a cycle is generated for controlling the charge output of a digital to analog converter to cancel out said charge of said pixel.
17. A method for digitally quantifying IR radiation impinging on a focal plane array as in claim 16 and wherein a cycle generated contains at least one ramp.
18. A method for digitally quantifying IR radiation impinging on a focal plane array as in claim 13, and wherein a tradeoff between the resolution of said quantification and the frame rate is enabled by programming said dual ramp conversion cycle.
19. A method for digitally quantifying IR radiation impinging on a focal plane array as in claim 13 and wherein said charge of said pixel is loaded first into a capacitor of an integrator wherein said integrator is connected to a column of said array, and wherein said charge is later digitally quantified.
20. A method for digitally quantifying IR radiation impinging on a focal plane array as in claim 13, and wherein the pixels of said array are grouped, such that quantification is performed simultaneously for all members of said group, each group at a time, allocating at one conversion time one ADC to each member of each group.
21. A method for digitally quantifying IR radiation impinging on a focal plane array as in claim 20, and wherein for said each group of pixels, one common ramp generator produces at least one linear voltage ramp whereby in each ADC a low level DC current is created by said ramp driving a capacitor connected to a virtual ground of an integrator.
22. A method for digitally quantifying IR radiation impinging on a focal plane array as in claim 20, and wherein each of said groups contain two rows of said array.
Type: Application
Filed: May 22, 2003
Publication Date: Apr 7, 2005
Inventors: Shimon Elkind (Carmiel), Amnon Adin (Haifa)
Application Number: 10/442,934