Voltage storage circuits
A voltage storage circuit, for use for example in an analogue-to-digital converter, includes an input switch element connected between an input node (IN) of the circuit and a first plate of a storage capacitor. The other plate of the capacitor is connected to a common terminal 3 of the circuit. A high-impedance amplifier element is connected to the first plate for providing at an output node (OUT) of the circuit an output voltage (V.sub.o) dependent upon the first plate potential (V.sub.c). The amplifier element has an FET input device whose gate electrode is connected to the first plate and whose source and drain electrode potentials are fixed in relation to the first plate potential (V.sub.c). Such a voltage storage circuit avoids charge injection to/from the amplifier element, with consequential charging/discharging of the storage capacitor, which would otherwise result from operation of the amplifier element
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Claims
2. A voltage storage circuit as claimed in claim 1, wherein the potential tracking device includes a current source, connected to the said first current-path electrode for causing the first current-path electrode potential to track the control electrode potential, and active follower means connected operatively between the said first and second current-path electrodes for causing the second current-path electrode potential to track the first current-path electrode potential.
3. A voltage storage circuit as claimed in claim 2, wherein the said electronic input device is an FET input transistor and the said control electrode is the gate electrode of the FET input transistor, the first current-path electrode is the source electrode of the FET input transistor, the said second current-path electrode is the drain electrode of the FET input transistor, and the said controllable current path is provided by the drain-source channel of the FET input transistor.
4. A voltage storage circuit as claimed in claim 3, wherein the said active follower means comprise a cascoding FET transistor connected with its drain-source channel in series with the drain-source channel of the said FET input transistor so that the source electrode potential of the cascoding transistor tracks the gate electrode potential thereof and also comprise a bias generator connected operatively between the source electrode of the FET input transistor and the gate electrode of the cascoding transistor for maintaining therebetween a substantially constant potential difference.
5. A voltage storage circuit as claimed in claim 2, wherein the said amplifier element is made up of first and second substantially identical circuit portions, the first portion including the said input device and the said active follower means and the second portion including the said current source.
6. A voltage storage circuit as claimed in claim 1, wherein the said input switch element is an electronic input switch element, operative in dependence upon the potential at a switching electrode thereof, the circuit further including switch driving means connected to cause the switching electrode potential to track the input terminal potential when the element is in its ON condition, thereby maintaining the switching electrode potential substantially fixed in relation to the input terminal potential, and operable to cause the switching electrode potential to change, relative to the input terminal potential such that the element is changed from its ON condition to its OFF condition.
7. A voltage storage circuit as claimed in claim 6, wherein the said switching electrode potential is derived from the said output signal.
8. A voltage storage circuit as claimed in claim 7, wherein the said switch driving means are connected operatively with the said output terminal and are operable, in dependence upon a switching signal received thereby, to apply to the said switching electrode either an ON potential, for maintaining the said input switch element in its ON condition, or an OFF potential, for maintaining the said input switch element in its OFF condition, the said ON and OFF potentials being each substantially fixed in relation to the said output terminal potential but differing from one another by a predetermined amount.
9. A voltage storage circuit as claimed in claim 8, having respective first and second biassing lines connected operatively to the said output terminal so as to be at potentials that are respectively fixed in relation to the output terminal potential, the second biassing line potential being equal to one of the said ON and OFF potentials and the potential difference between the said first and second biassing lines being greater than or equal to the said predetermined amount, wherein the said switch driving means include a bootstrap capacitor one plate of which is connected to the said switching electrode for providing the said switching electrode potential and also include connecting means connected with both plates of the bootstrap capacitor and with the said biassing lines and switchable, when the switching electrode potential is to be changed from the said one of its ON and OFF potentials to the other of those potentials, from a charging configuration, serving to connect the said one plate of the bootstrap capacitor to the said second biassing line whilst connecting the other plate thereof to the said first biassing line, to a floating configuration serving to isolate the said one plate from the second biasing line whilst connecting the said other plate to the said second biassing line, thereby to cause the potential at the said one plate to be changed from the second biassing line potential to a potential differing therefrom by the aid predetermined amount.
10. A voltage storage circuit as claimed in claim 8, having respective first, second and third biasing lines connected operatively to the said output terminal so as to be at potentials that are respectively fixed in ration to the output tern potential, the third biassing line potential being equal to one of the said ON and OFF potentials and the potential difference between the said first and second biassing lines being greater than or equal to the said predetermined amount, wherein the said switch driving means include a bootstrap capacitor one plate of which is connected to the said switching electrode for providing the said switching electrode potential and also include connecting means connected with both plates of the bootstrap capacitor and with the said biassing lines and switchable, when the switching electrode potential is to be changed from the said one of its ON and OFF potentials to the other of those potentials, from a changing configuration, serving to connect the said one plate of the bootstrap capacitor to the said third biassing line whilst connecting the other plate thereof to the said first biassing line, to a floating configuration serving to isolate the said one plate from the third biassing line whilst connecting the said other plate to the said second biassing line, thereby to cause the potential at the said one plate to be changed from the third biassing line potential to a potential differing therefrom by the said predetermined amount.
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Type: Grant
Filed: Jan 23, 1997
Date of Patent: Dec 29, 1998
Assignee: Fujitsu Limited (Kawasaki)
Inventor: Ian J. Dedic (Northolt)
Primary Examiner: Brian K. Young
Law Firm: Staas & Halsey
Application Number: 8/788,030
International Classification: H03M 112;
