BUFFER WITH GAIN SELECTION

Abstract

An electronic device has an amplifier having an amplifier input terminal and an amplifier output terminal, the amplifier output terminal being connected to the device output terminal. An input capacitor is connected between the device input terminal and the amplifier input terminal. A feedback capacitor is connected between the amplifier output terminal and the amplifier input terminal. A switchable capacitor has a first terminal connected to the amplifier input terminal and a second terminal connected to a respective first terminal of each of a first switch and a second switch. The first switch has its second terminal connected to the device input terminal. The second switch has its second terminal connected to the amplifier output terminal. In this arrangement, the switchable capacitor can be switched between forming part of the input path of the amplifier or the feedback path of the amplifier.

Claims

1. An electronic device having a device input terminal and a device output terminal, the device comprising: an amplifier having an amplifier input terminal and an amplifier output terminal, said amplifier output terminal being connected to the device output terminal; an input capacitor having a first terminal thereof connected to the device input terminal, and a second terminal thereof connected to the amplifier input terminal; a feedback capacitor having a first terminal thereof connected to the amplifier output terminal, and a second terminal thereof connected to the amplifier input terminal; and a switchable capacitor having a first terminal thereof connected to the amplifier input terminal, and a second terminal thereof connected to a respective first terminal of each of a first switch, a second switch, and a third switch; wherein the first switch has a second terminal thereof connected to the device input terminal; wherein the second switch has a second terminal thereof connected to the amplifier output terminal; and wherein the third switch has a second terminal thereof connected to ground or virtual ground, such that the switchable capacitor may be disconnected altogether when it is not required.

2. The electronic device as claimed in claim 1, further comprising: a second switchable capacitor having a first terminal thereof connected to the amplifier input terminal, and a second terminal thereof connected to a respective first terminal of each of a fourth switch and a fifth switch; wherein the fourth switch has a second terminal thereof connected to the device input terminal; and wherein the fifth switch has a second terminal thereof connected to the amplifier output terminal.

3. The electronic device as claimed in claim 2, further comprising: a third switchable capacitor having a first terminal thereof connected to the amplifier input terminal, and a second terminal thereof connected to the respective first terminals of the fourth and fifth switches.

4. The electronic device as claimed in claim 2, wherein the second terminal of the second switchable capacitor is connected to a first terminal of a sixth switch, wherein a second terminal of said sixth switch is connected to a second predetermined reference voltage.

5. The electronic device as claimed in claim 4, wherein the second predetermined reference voltage is ground or virtual ground.

6. The electronic device as claimed in claim 1, wherein each of the capacitors has a respective capacitance equal to a respective integer multiple of a unit capacitance.

7. The electronic device as claimed in claim 1, further comprising: a first sampling switch having a first terminal thereof connected to the amplifier input terminal and a second terminal thereof connected to a third predetermined reference voltage; a second sampling switch having a first terminal thereof connected to the amplifier output terminal and a second terminal thereof connected to a fourth predetermined reference voltage; and a third sampling switch having a first terminal thereof connected to the device input terminal and a second terminal thereof connected to the first terminal of the input capacitor.

8. The electronic device as claimed in claim 7, wherein the first, second, and third sampling switches are controlled or controllable such that when said switches are disabled, the first sampling switch is disabled before the second and third sampling switches are disabled.

9. The electronic device as claimed in claim 1, further comprising: a first amplifier enable switch having a first terminal thereof connected to the amplifier output terminal and a second terminal thereof connected to the second terminal of the feedback capacitor; and a second amplifier enable switch having a first terminal thereof connected to the first terminal of the input capacitor.

10. The electronic device as claimed in claim 9, wherein a second terminal of the second amplifier enable switch is connected to a fifth predetermined reference voltage.

11. The electronic device as claimed in claim 10, wherein the fifth predetermined reference voltage is ground or virtual ground.

12. The electronic device as claimed in claim 1, wherein the electronic device has a second device input terminal and a second device output terminal, wherein: the amplifier has a second amplifier input terminal and a second amplifier output terminal, said second amplifier output terminal being connected to the second device output terminal; wherein the electronic device further comprises: a second input capacitor having a first terminal thereof connected to the second device input terminal, and a second terminal thereof connected to the second amplifier input terminal; a second feedback capacitor having a first terminal thereof connected to the second amplifier output terminal, and a second terminal thereof connected to the second amplifier input terminal; and a fourth switchable capacitor having a first terminal thereof connected to the second amplifier input terminal, and a second terminal thereof connected to a respective first terminal of each of a seventh switch and an eighth switch; wherein the seventh switch has a second terminal thereof connected to the second device input terminal; and wherein the eighth switch has a second terminal thereof connected to the second amplifier output terminal.

13. The electronic device as claimed in claim 12, wherein: the first amplifier input is an inverting input; the second amplifier input is a non-inverting input; the first amplifier output is a non-inverting output; and the second amplifier output is an inverting output.

14. The electronic device as claimed in claim 1, wherein one or more of the capacitors respectively comprises a metal capacitor.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0064] Certain embodiments of the invention will now be described, by way of non-limiting example only, with reference to the accompanying drawings in which:

[0065] FIG. 1 is a schematic diagram of an electronic device with a buffer circuit in accordance with an embodiment of the present invention;

[0066] FIG. 2 is a schematic diagram of a genericised version of an electronic device in accordance with an embodiment of the present invention;

[0067] FIG. 3 is a schematic diagram of a single-ended configuration of a genericised electronic device in accordance with an embodiment of the present invention;

[0068] FIG. 4 is a plot illustrating the PSRR improvements provided by embodiments of the present invention; and

[0069] FIG. 5 is a schematic diagram of an electronic device with a differential configuration.

DETAILED DESCRIPTION

[0070] FIG. 1 is a schematic diagram of an electronic device 100 with a buffer circuit in accordance with an embodiment of the present invention. The device 100 is suitable for use in a sample-and-hold circuit, such as in an ADC. The electronic device 100 of FIG. 1 is arranged to receive an input signal at a device input terminal 102, and to produce a buffered version of the signal at an output terminal 104 of the device.

[0071] The device 100 includes an amplifier 106, which has an inverting input terminal 108, a non-inverting input terminal 110, a non-inverting output terminal 112, and an inverting input terminal 114. As such, the amplifier 106 can be used as a fully differential amplifier. Alternatively, the amplifier 106 can be configured in a single-sided mode, as is shown in FIG. 1. A differential arrangement is shown in FIG. 5, which is discussed in detail further below.

[0072] An input capacitor 116 is arranged in the input path of the amplifier 106, such that one terminal of the input capacitor 116 is connected to the device input terminal 102 (via a switch, outlined below), and the other terminal of the input capacitor 116 is connected to the inverting input terminal 108 of the amplifier 106.

[0073] A feedback capacitor 120 arranged in the feedback path of the amplifier 106, such that one terminal of the feedback capacitor 120 is connected to the non-inverting output terminal 112 of the amplifier 106, and the other terminal of the feedback capacitor 120 is connected to the inverting input terminal 108 of the amplifier 106.

[0074] The device 100 also comprises three switchable capacitors 122, 124, 126, which are described in further detail below.

[0075] The first switchable capacitor 122 is arranged such that one of its terminals is connected to the inverting input terminal of the amplifier 106, and such that its other terminal is connected to respective first terminals of a first switch 128, a second switch 130, and a third switch 132. The first switch 128 has its other terminal connected to the device input terminal 102 (via a switch 144, as outlined below). The second switch 130 has its other terminal connected to the non-inverting output terminal 112 of the amplifier 106. The third switch 132 has its other terminal connected to ground (or virtual ground).

[0076] The second and third switchable capacitors 124, 126 are arranged in parallel, with the first terminals of each connected together and to the inverting input 108 of the amplifier 106 (and thus also to the input capacitor 116, first switchable capacitor 122, and feedback capacitor 120). The other terminals of the second and third switchable capacitors 124, 126 are connected together and to respective first terminals of a fourth switch 134, a fifth switch 136, and a sixth switch 138.

[0077] The fourth switch 134 has its other terminal connected to the device input terminal 102. The fifth switch 136 has its other terminal connected to the non-inverting output terminal 112 of the amplifier 106 (and therefore to the second terminal of the second switch 130). The sixth switch 138 has its other terminal connected to ground (or virtual ground).

[0078] The device 100 also includes three sampling switches 140, 142, 144. The first sampling switch 140 is connected between the inverting input terminal 108 of the amplifier 106 and ground (or virtual ground). The second sampling switch 142 is connected between the non-inverting output terminal 112 of the amplifier 106 and ground (or virtual ground). The third sampling switch 144 is connected between the device input terminal 102 and the first terminal of the input capacitor 116.

[0079] Additionally, the device 100 includes two amplifier enable switches 146, 148. The first amplifier enable switch 146 is connected between the non-inverting output terminal 112 of the amplifier 106 and the second terminal of the feedback capacitor 120. The second amplifier enable switch 148 is connected to the first terminal of the input capacitor 116, and the other terminal of the second amplifier enable switch 148 may be connected to ground (or virtual ground) when the device 100 is used in a single-ended configuration, as is shown in FIG. 3.

[0080] Each of the capacitors—the input capacitor 116, feedback capacitor 120, and switchable capacitors 122, 124, 126—is, in this particular embodiment, a unit capacitance. That is to say, all of these capacitors have the same capacitance ‘C’. However, these could be different, where each has a capacitance value equal to an integer multiple of C, e.g. 2C, 3C, 4C, etc. Thus, for example, if the unit capacitance C were equal to 100 μF, a capacitor having a capacitance of 3C would have a capacitance of 300 μF.

[0081] Depending on the particular combination of switches closed at any given time, the ratio of the feedback capacitance to the input capacitance can be varied. This is because the switchable capacitors 122, 124, 126 may each be connected to form part of the input capacitance ‘C1’, the feedback capacitance ‘C2’, or removed from the circuit such that they contribute to neither the input capacitance C1 nor the feedback capacitance C2.

[0082] The control of the switches 128, 130, 132, 134, 136, 138 that adjust the capacitor ratio may be effected by a control logic (not shown for ease of illustration) that supplies control signals to a control terminal of each of the switches. Operation of the switches that control a particular capacitor are exclusive—only one may be enabled at any given time. That is to say, only one of the first through third switches 128, 130, 132 may be enabled (i.e. closed) simultaneously; and only one of the fourth through sixth switches 134, 136, 138 may be enabled (i.e. closed) simultaneously.

[0083] For example, when a ratio C1:C2=2:3 is desired, the first switch 128 and fifth switch 136 are closed, and the other switches 130, 132, 134, 138 are opened. In this mode, all three of the switchable capacitors 122, 126, 128 are in use, and so the third and sixth switches 132, 138 are opened and no defined potential is needed because there would be no floating capacitors. With this arrangement, the first switchable capacitor 122 forms part of the input capacitance, adding to the input capacitor 116, leading to an input capacitance C1 of 2*C. Conversely, the second and third switchable capacitors 126, 128 form part of the feedback capacitance, adding to the feedback capacitor 120, leading to a feedback capacitance C2 of 3*C. This therefore provides the desire C1:C2=2:3 ratio.

[0084] As another example, if unity gain is desired (i.e. C1:C2=1:1), the third and sixth switches 132, 138 are closed, and the other switches 128, 130, 134, 136 are opened. As the switchable capacitors 122, 126, 128 are not in use for this gain mode, closing the third and sixth switches 132, 138 connects the switchable capacitors 122, 126, 128 to a defined potential (ground or virtual ground) to prevent floating capacitors. In this configuration, only the input capacitor 116 contributes to the input capacitance C1, and only the feedback capacitor 120 contributes to the feedback capacitance C2. As these are equal (both having the unit capacitance C), this provides the desired unity gain ratio C1:C2=1:1.

[0085] In another example, a ratio of C1:C2=2:1 may be desired. To achieve this, the first switch 128 and sixth switch 138 may be closed, and the second through fifth switches 130, 132, 134, 136 are opened. This causes the first switchable capacitor 122 to add to the input capacitance alongside the input capacitor 116 (summing to 2*C), while only the feedback capacitor 120 contributes to the feedback capacitance (which is therefore 1*C), leading to the desired 2:1 ratio for C1:C2. By closing the sixth switch 138, the second and third switchable capacitors 126, 128 are not left floating, but are instead connected to a defined potential (i.e. ground or virtual ground).

[0086] Similar concept can be applied to gain modes of 4:1 (by closing only the first and fourth switches 128, 134) and 1:2 (by closing only the second and sixth switches 130, 138).

[0087] The control logic (or some other control logic) may also control operation of the three sampling switches 140, 142, 144 and the two amplifier enable switches 146, 148.

[0088] The sampling switches 140, 142, 144 are controlled or controllable such that the first sampling switch is disabled before the second and third sampling switches are disabled, i.e. there is a delay before the second and third sampling switches are disconnected following disconnection of the first sampling switch. The delay between opening the first sampling switch 140 and opening the second and third sampling switches 142, 144 is used in order to avoid introducing unwanted charge injection when the second and third sampling switches 142, 144 are opened. Using the bottom plate sampling technique, known in the art per se also helps to address this issue.

[0089] The respective control signals applied to the first sampling switch 140 and to the amplifier enable switches 146, 148 are non-overlapping clocks.

[0090] FIG. 2 is a schematic diagram of a genericised version of an electronic device 200 in accordance with an embodiment of the present invention. Elements having a reference numeral starting with a ‘2’ correspond in structure and function to those elements having the same reference numeral starting with al′ in FIG. 1, unless context dictates otherwise. In other words, an element with reference numeral ‘2xx’ in FIG. 2 corresponds to the element having reference numeral ‘1xx’ in FIG. 1.

[0091] As shown in FIG. 2, a generalised switchable capacitor block 250 comprising a switchable capacitor 252 may be configured such that it can contribute to either the input capacitance (by enabling a first switch 254), the feedback capacitance (by enabling a second switch 256), or neither (by enabling a third switch 258). As before, operation of the switches 254, 256, 258 surrounding the capacitor 252 are exclusive—only one may be enabled at any given time.

[0092] Any number of these switchable capacitor blocks 250 may be included, connected in parallel with one another, such that each block can be configured independently to form part of the input or feedback capacitance, or to be disconnected.

[0093] The switchable capacitor 252 may be a fixed capacitor, or it may be a variable capacitor, implemented e.g. using a capacitor array (i.e. a matrix of capacitors, a subset of which can be enabled for a given capacitance).

[0094] FIG. 3 is a schematic diagram of a single-ended configuration genericised electronic device 300 in accordance with an embodiment of the present invention. Elements having a reference numeral starting with a ‘3’ correspond in structure and function to those elements having the same reference numeral starting with al′ in FIG. 1 and/or with a ‘2’ in FIG. 2, unless context dictates otherwise. In other words, an element with reference numeral ‘3xx’ in FIG. 3 corresponds to the element having reference numeral ‘1xx’ in FIG. 1 and/or ‘2xx’ in FIG. 2.

[0095] Unlike in FIGS. 1 and 2, in which the amplifier 106, 206 may be arranged in a fully differential configuration (shown in full detail in FIG. 5), in the device 300 of FIG. 3, the amplifier 306 is arranged in a single-ended configuration. In this configuration, the non-inverting input 310 of the amplifier 306 is connected to ground (or virtual ground), and only a single ended output 312 is taken from the amplifier 306. The amplifier enable switch 348 is connected between the input-side of the input capacitor 316 and ground (or virtual ground).

[0096] FIG. 4 is a plot illustrating the PSRR improvements provided by embodiments of the present invention. The y-axis shows the inverse of PSRR (i.e. a lower value on the y-axis indicates better PSRR performance), and the x-axis shows frequency. The dotted dashed lines are the PSRR simulation results for a conventional device, known in the art per se, with gain modes of 1/1, 2/1, 4/1, and 1/2 using a Monte Carlo statistical mismatch corner. This conventional device used for comparison does not have the ability to terminate the capacitors which are not in use, i.e. they are left floating. By comparison, the solid lines are the PSRR simulation results for the device of FIG. 1 using gain modes of 1/1, 2/1, 4/1, and 1/2 when using the invented Cap Array design with sw3 and sw6 used to terminate the capacitors which are not in use depending on gain mode setting.

[0097] As can be seen in FIG. 4, devices embodying the present invention achieve significant improvements in PSRR at frequencies <100 Hz, i.e. the plots are shifted downwards. With an improved PSRR, the device also offers better immunity when used within an ADC operated at a high resolution mode, e.g. a 12-bit or 14-bit ADC. At certain gain mode conditions, the device shows a 23 dB PSRR improvement when compared to a conventional device, known in the art per se.

[0098] FIG. 5 is a schematic diagram of an electronic device 500 with a differential configuration, in accordance with a further embodiment of the present invention. Elements having a reference numeral starting with a ‘5’ correspond in structure and function to those elements having the same reference numeral starting with a ‘1’ in FIG. 1, and/or with a ‘2’ in FIG. 2, and/or with a ‘3’ in FIG. 3, unless context dictates otherwise. In other words, an element with reference numeral ‘5xx’ in FIG. 5 corresponds to the element having reference numeral ‘1xx’ in FIG. 1; ‘2xx’ in FIG. 2; and/or ‘3xx’ in FIG. 3.

[0099] For differential operation, the circuit that connected between the inverting input terminal 508 and non-inverting output terminal 512 of the amplifier 506 in FIG. 5 is duplicated, where the duplicate circuit is connected between the non-inverting input terminal 510 and inverting output terminal 514 of the amplifier 506.

[0100] In this differential configuration, the device 500 makes use of a further device input and device output, such that the device input 502 takes a voltage across two terminals, where one receives the negative input IN− (which is supplied to the inverting amplifier input 508 as before) and the other receives the positive input IN+(which is supplied to the non-inverting amplifier input 510). The output is taken across the device output 504 as a positive output OUT+(taken from the non-inverting amplifier output 512) and a negative output OUT− (taken from the inverting amplifier output 514).

[0101] The input-side terminals of the input capacitors 516 on each of the negative and positive sides of the device are connected together via a pair of amplifier enable switches 548. The respective first sampling switches 540 for each of the negative and positive sides of the device are connected together at a ground node.

[0102] A generalised switchable capacitor block 550 is provided for each of the positive and negative sides of the amplifier 506. Each such block 550 comprises a switchable capacitor 552 that may be configured such that it can contribute to either the input capacitance (by enabling a first switch 554), the feedback capacitance (by enabling a second switch 556), or neither (by enabling a third switch 558). As before, operation of the switches 554, 556, 558 surrounding the capacitor 252 are exclusive—only one may be enabled at any given time.

[0103] Any number of these switchable capacitor blocks 550 may be included as discussed previously, however there will generally be the same number of blocks 550 on the negative side as on the positive side.

[0104] Control of the various sampling switches 540, 542, 544 and amplifier enable switches 546, 548 is as discussed previously, where each of the switches on the negative side of the device are generally controlled in synchronisation with the corresponding switches (i.e. those having the same reference number) on the positive side of the device.

[0105] Thus it will be appreciated that embodiments of the present invention provide an improved buffer device for use in sample-and-hold circuits in which a switchable capacitor may be switched between being part of the input path or the feedback path of an amplifier, allowing the capacitance ratio to be changed accordingly. This scheme may provide for a significant reduction in the number of capacitors required to achieve a configurable gain ratio. Furthermore, embodiments of the present invention may provide for significant improvements in PSRR performance by providing a mechanism through which capacitors that are not needed can be tied to a predetermined level so as to not be left floating.

[0106] While specific embodiments of the present invention have been described in detail, it will be appreciated by those skilled in the art that the embodiments described in detail are not limiting on the scope of the claimed invention.