US20110221520A1 - Switched capacitor amplifier circuit with clamping - Google Patents
Switched capacitor amplifier circuit with clamping Download PDFInfo
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- US20110221520A1 US20110221520A1 US12/722,814 US72281410A US2011221520A1 US 20110221520 A1 US20110221520 A1 US 20110221520A1 US 72281410 A US72281410 A US 72281410A US 2011221520 A1 US2011221520 A1 US 2011221520A1
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- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03F—AMPLIFIERS
- H03F3/00—Amplifiers with only discharge tubes or only semiconductor devices as amplifying elements
- H03F3/005—Amplifiers with only discharge tubes or only semiconductor devices as amplifying elements using switched capacitors, e.g. dynamic amplifiers; using switched capacitors as resistors in differential amplifiers
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- the present disclosure relates to switched capacitor amplifier circuits and more particularly to switched capacitor amplifier circuits with clamping.
- Switched capacitor amplifier circuits may be used in a wide variety of applications. Switched capacitor amplifier circuits may encounter gain errors due to a signal overdrive condition. When an operational transconductance amplifier (OTA) in the switched-capacitor amplifier circuit is overdriven, it may require several clock cycles to return to a desired operating point. During the intervening recovery period, output signal errors may occur. Some conventional switched capacitor amplifier circuits clamp an input signal, which may introduce signal distortion in the desired signal range.
- OTA operational transconductance amplifier
- a system comprises a switched capacitor amplifier including an operational amplifier (opamp).
- a switching circuit comprises a first switch connected across inputs of the opamp.
- a second switch is connected across outputs of the opamp.
- An overdrive detect circuit communicates with the first and second switches and selectively shorts the inputs and the outputs of the opamp when the input voltage is greater than a first predetermined overdrive voltage or when the input voltage is less than a second predetermined overdrive voltage.
- the switched capacitor amplifier comprises a first plurality of capacitors having first terminals that communicate with one of the inputs of the opamp.
- a second plurality of capacitors includes first terminals that communicate with another one of the inputs of the opamp.
- the switched capacitor amplifier further comprises a first plurality of switches that selectively connect second terminals of the first plurality of capacitors to one of a positive terminal of the input voltage, a positive terminal of a reference voltage and a negative terminal of a reference voltage.
- a second plurality of switches selectively connect second terminals of the second plurality of capacitors to one of a negative terminal of the input voltage, the positive terminal of the reference voltage and the negative terminal of the reference voltage.
- the switched capacitor amplifier comprises a first circuit that receives an input signal and that generates switch control signals for the first and second plurality of switches and a first code signal.
- the first circuit comprises a plurality of comparators that receive a plurality of voltage references, respectively, and the input voltage.
- a decoder generates switching signals for the first and second plurality of switches and the first code signal based on outputs of the plurality of comparators.
- the switched capacitor amplifier comprises a first capacitor including a first terminal connected to one of the inputs of the opamp and a second terminal connected to one of the outputs of the opamp.
- a second capacitor includes a first terminal connected to another one of the inputs of the opamp and a second terminal connected to another one of the outputs of the opamp.
- the overdrive detect circuit comprises a first comparator including one input that receives the first predetermined overdrive voltage and another input that receives the input voltage.
- a second comparator includes one input that receives the second predetermined overdrive voltage and another input that receives the input voltage.
- a logic gate receives outputs of the first and second comparators and generates an output that controls the switching circuit.
- the first predetermined overdrive voltage is a positive voltage and the second predetermined overdrive voltage is a negative voltage.
- a pipelined analog to digital converter comprises a plurality of stages.
- a first one of the plurality of stages comprises the system.
- a flash analog to digital converter communicates with a last one of the plurality of stages.
- An alignment and digital error correction circuit communicates with the plurality of stages and the flash analog to digital converter.
- FIG. 1 is a functional block diagram of a pipelined ADC according to the present disclosure
- FIG. 2 is a functional block diagram of one of the stages of the pipelined ADC according to the present disclosure
- FIG. 3 is a functional block diagram of a switched capacitor amplifier circuit according to the present disclosure
- FIGS. 4A and 4B are functional block diagrams and electrical schematics of the switched capacitor amplifier circuit of FIG. 3 in a sampling phase;
- FIG. 5 is a functional block diagram and electrical schematic of the switched capacitor amplifier circuit of FIG. 4A in an amplifying phase
- FIG. 6 is a functional block diagram and electrical schematic of an exemplary implementation of a comparator circuit
- FIG. 7 is a functional block diagram and electrical schematic of an exemplary overdrive detect circuit
- FIG. 8 is a graph illustrating residue voltage as a function of input voltage for an exemplary implementation.
- FIG. 9 is a chart relating input voltages, MDAC and OD codes, residue voltages, switching, and output codes for the exemplary implementation of FIG. 8 .
- a clamping circuit according to the present disclosure provides improved overdrive recovery even when overdrive conditions occur frequently.
- the quick overdrive recovery tends to enhance system performance.
- the switch capacitor amplifier circuit with clamping according to the present disclosure will be described below in conjunction with a multiplying digital to analog converter (MDAC) stage of a pipelined analog to digital converter (ADC).
- MDAC multiplying digital to analog converter
- ADC pipelined analog to digital converter
- skilled artisans will appreciate that the switched capacitor amplifier circuit according to the present disclosure may be used in other circuits.
- a pipelined analog to digital converter (ADC) 10 includes multiple stages 12 - 1 , 12 - 2 , 12 - 3 , . . . , and 12 -N (collectively stages 12 ). Each of the stages 12 is used to determine subsets of the bits. The last stage 12 -N communicates with a flash ADC 14 . The stages 12 and the flash ADC 14 communicate with an alignment and digital error correction circuit 16 , which corrects for latency caused by the delays of the stages 12 .
- the pipelined ADC 10 converts the analog input signal to a T bit output, where T and N are integers and T>N. For example only, T may be set equal to 12 and N may be set equal to 4 in some implementations.
- Each of the stages 12 includes a sample and hold circuit 24 , a flash ADC 26 , a digital to analog converter (DAC) 30 , a summing circuit 32 and an amplifier circuit 34 .
- the sample and hold circuit 24 , the DAC 30 , the summing circuit 32 and the amplifier circuit 34 in the stages 12 may be implemented by a MDAC with a switched capacitor amplifier circuit according to the present disclosure.
- the analog input V in is sampled and held by the sample and hold circuit 24 .
- the flash ADC 26 quantizes the analog input V in to M bits, where M may be set equal to T/N. For example only, M may be set equal to 3 in some implementations.
- the output of the flash ADC 26 is then fed to the DAC 30 (which may be accurate to about T bits).
- the analog output is subtracted from the input at the summing circuit 32 .
- the residue voltage is amplified by the amplifier circuit 34 and fed to the stage 12 - 2 .
- the amplifier 34 may have a gain. For example only, the gain may be set to 2 in some implementations.
- the gained-up residue voltage continues to another stage of the pipeline ADC 10 , which provides M bits per stage.
- the flash ADC 14 may resolve some of the LSB bits. For example only, the flash ADC 14 may resolve 4 LSBs in some implementations.
- the M bits from each stage 12 are determined at different points in time, all of the bits corresponding to the same sample of V in are time-aligned by shift registers before being fed to time alignment and digital error correction circuit 16 .
- the stage 12 can start processing the next sample received from the sample and hold circuit 24 of the stage 12 . This pipelining action improves throughput.
- the switched capacitor amplifier circuit 110 includes sampling capacitors C s .
- the number of sampling capacitors may be set equal to 2*2 M .
- M 2 and 8 capacitors are provided, including C s1p , C s2p , C s3p and C s4p and C s1m , C s2m , C s3m and C s4m .
- the switched capacitor amplifier 110 further includes switches SW 1p , SW 2p , SW 3p and SW 4p that selectively switch terminals of the capacitors C s1p , C s2p , C s3p and C s4p to a positive or negative terminal of a reference voltage V ref or a positive terminal of an input voltage V in .
- the switched capacitor amplifier 110 further includes switches SW 1m , SW 2m , SW 3m and SW 4m that selectively switch the capacitors C s1m , C s2m , C s3m and C s4m to the positive or negative terminal of the reference voltage V ref or a negative terminal of the input voltage V in .
- the switched capacitor amplifier 110 further includes a multiplying DAC (MDAC) comparator circuit 120 that receives the input voltage V in and that generates an MDAC code signal and switch control signals for switching the switches SW 1m , SW 2m , SW 3m and SW 4m and SW 1p , SW 2p , SW 3p and SW 4p .
- the switched capacitor amplifier 110 further includes an overdrive OD detect circuit 130 that receives the input voltage V in and that generates an OD code signal and switch control signals for clamping switches SW CLPin and SW CLPout .
- MDAC multiplying DAC
- the switched capacitor amplifier 110 further includes an operational transconductance amplifier (OTA) 140 .
- the OTA 140 includes a non-inverting input that communicates with second terminals of the capacitors C s1p , C s2p , C s3p and C s4p and an inverting input that communicates with second terminals of the capacitors C s1m , C s2m , C s3m and C s4m .
- Feedback capacitor C fp communicates with the non-inverting input and output of the OTA 140 .
- Feedback capacitor C fn communicates with the inverting input and output of the OTA 140 .
- Input clamping switch SW CLPin communicates with the non-inverting and inverting inputs of the OTA 140 .
- Output clamping switch SW CLPout communicates with the non-inverting and inverting outputs of the OTA 140 .
- a clamping circuit according to the present disclosure provides overdrive recovery (the next clock cycle gain phase provides full accuracy) without distorting the signal and with minimal power dissipation.
- the switch control signals from the MDAC comparator circuit 120 connect first terminals of the capacitors C s1p , C s2p , C s3p and C s4p to a positive terminal of the input signal V.
- the switch control signals from the MDAC comparator circuit 120 connect the first terminals of the capacitors C s1m , C s2m , C s3m and C s4m to a negative terminal of the input signal V in .
- the overdrive detect circuit 130 generates the switch control signals to position the switches SW CLPin and SW CLPout in closed positions, which effectively shorts the OTA 140 (omitted from FIG. 4A ).
- the input signal V in is sampled.
- the input signal V in is applied across the input sampling capacitors C s1p , C s2p , C s3p and C s4p and C s1m , C s2m , C s3m and C s4m while the feedback capacitors C fp and C fn are reset to zero.
- the MDAC comparator circuit 120 makes a quantizer decision.
- the overdrive detect circuit 130 detects whether or not the signal exceeds overdrive thresholds.
- the OTA 140 may be active as the amplifier for a subsequent MDAC stage. In other words, the OTA 140 may be switched and shared with another stage. For example only in FIG. 4B , switches S 1 to S 4 can be used to switch the OTA 140 between the stages 12 that are operating in opposite phases. As can be appreciated, other components from FIG. 4A are omitted for purposes of clarity.
- the switched capacitor amplifier circuit 110 is shown in an amplifying phase.
- the switch control signals from the MDAC comparator circuit 120 position the switches SW 1p , SW 2p , SW 3p , SW 4p , SW 1m , SW 2m , SW 3m and SW 4m based on the quantizer decision to connect the first terminals of the capacitors C s1p , C s2p , C s3p , C s4p , C s1m , C s2m , C s3m and C s4m to the positive or negative terminal of the reference voltage V ref .
- the clamping switches SW CLPin and SW CLPout may be opened or closed as will be described below.
- the capacitors C s1m , C s2m , C s3m and C s4m and C s1p , C s2p , C s3p and C s4p are connected to reference potentials in a configuration that is determined by the quantizer decision. If an overdrive condition is detected, the clamp switches SW CLPin and SW CLPout are closed. If the overdrive condition is not detected when the input signal V in was sampled, the clamp switches SW CLPin and SW CLPout remain open and charge is transferred to satisfy forcing potentials (V ref +/ ⁇ ) and OTA gain, which results in a residue output V OUT . This is the normal operation that occurs when the overdrive detection and clamping are not performed.
- the clamp switches SW CLPin and SW CLPout are used when clamping is needed and additional switches (not shown) may be placed in parallel with SW CLPin and SW CLPout .
- the additional switches may be low impedance switches that are closed during the sampling phase (instead of the clamp switches SW CLPin and SW CLPout ) and opened otherwise. Additional switches may also be used to reset the capacitors C fp and C fm .
- the clamp switch SW CLPout forces the output to zero since random offsets in the OTA 140 would otherwise force the output into saturation since the feedback loop is broken. Since the OTA 140 is maintained at a valid operating point, it does not need to recover from an overdrive condition and will provide the nominal gain accuracy on the next clock cycle.
- the switches SW CLPin and SW CLPout can be very small and have negligible effect on normal operation.
- the overdrive detection circuit 130 also provides a digital output which allows the alignment digital error correction circuit 16 to properly clamp the digital output. Since the switch SW CLPout forces the MDAC output to zero, the composite output at the end of the pipeline has a digital value near full scale. Due to analog offsets, the digital output might be less than full scale, which would appear as a valid signal.
- the OD code output by the overdrive detection circuit 130 is used to force digital clamping to full scale.
- the MDAC comparator circuit 120 includes comparators 150 - 1 , 150 - 2 , 150 - 3 and 150 - 4 (collectively comparators 150 ) that receive the input voltage V in and reference voltages.
- the input voltage V in is connected to the inverting input and the non-inverting inputs receive ⁇ 0.75V and ⁇ 0.25V, respectively.
- the comparators 150 - 3 and 150 - 4 the input voltage V in is connected to the non-inverting input and the inverting inputs receive +0.25V and +0.75V, respectively.
- Outputs of the comparators 150 are input to a decoder 152 , which provides a 2-bit output 154 that will be described further in FIGS. 8 and 9 .
- the overdrive detect circuit 130 includes comparators 156 - 1 and 156 - 2 that receive the input voltage V in at inverting and non-inverting inputs, respectively.
- the comparators 156 - 1 and 156 - 2 receive reference voltages of ⁇ 1.25V and +1.25V at non-inverting and inverting inputs, respectively.
- Outputs of the comparators 156 are input to a logic gate 170 such as an OR gate.
- An output 172 of the logic gate 170 generates the OD code.
- FIGS. 8 and 9 additional operating details for the exemplary implementation are shown.
- residue voltage is shown as a function of input voltage.
- FIG. 9 input voltage ranges, MDAC and OD codes, capacitor switching details, residue voltages and output codes are shown.
- the stage 12 with clamping may be the first stage of the pipelined analog to digital converter.
- the subsequent stages 12 do not need to have clamp circuits since the overdrive voltage will be handled by the first stage.
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Abstract
Description
- The present disclosure relates to switched capacitor amplifier circuits and more particularly to switched capacitor amplifier circuits with clamping.
- The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
- Switched capacitor amplifier circuits may be used in a wide variety of applications. Switched capacitor amplifier circuits may encounter gain errors due to a signal overdrive condition. When an operational transconductance amplifier (OTA) in the switched-capacitor amplifier circuit is overdriven, it may require several clock cycles to return to a desired operating point. During the intervening recovery period, output signal errors may occur. Some conventional switched capacitor amplifier circuits clamp an input signal, which may introduce signal distortion in the desired signal range.
- A system comprises a switched capacitor amplifier including an operational amplifier (opamp). A switching circuit comprises a first switch connected across inputs of the opamp. A second switch is connected across outputs of the opamp. An overdrive detect circuit communicates with the first and second switches and selectively shorts the inputs and the outputs of the opamp when the input voltage is greater than a first predetermined overdrive voltage or when the input voltage is less than a second predetermined overdrive voltage.
- In other features, the switched capacitor amplifier comprises a first plurality of capacitors having first terminals that communicate with one of the inputs of the opamp. A second plurality of capacitors includes first terminals that communicate with another one of the inputs of the opamp. The switched capacitor amplifier further comprises a first plurality of switches that selectively connect second terminals of the first plurality of capacitors to one of a positive terminal of the input voltage, a positive terminal of a reference voltage and a negative terminal of a reference voltage. A second plurality of switches selectively connect second terminals of the second plurality of capacitors to one of a negative terminal of the input voltage, the positive terminal of the reference voltage and the negative terminal of the reference voltage.
- In other features, the switched capacitor amplifier comprises a first circuit that receives an input signal and that generates switch control signals for the first and second plurality of switches and a first code signal. The first circuit comprises a plurality of comparators that receive a plurality of voltage references, respectively, and the input voltage. A decoder generates switching signals for the first and second plurality of switches and the first code signal based on outputs of the plurality of comparators.
- In other features, the switched capacitor amplifier comprises a first capacitor including a first terminal connected to one of the inputs of the opamp and a second terminal connected to one of the outputs of the opamp. A second capacitor includes a first terminal connected to another one of the inputs of the opamp and a second terminal connected to another one of the outputs of the opamp. The overdrive detect circuit comprises a first comparator including one input that receives the first predetermined overdrive voltage and another input that receives the input voltage. A second comparator includes one input that receives the second predetermined overdrive voltage and another input that receives the input voltage. A logic gate receives outputs of the first and second comparators and generates an output that controls the switching circuit.
- In other features, the first predetermined overdrive voltage is a positive voltage and the second predetermined overdrive voltage is a negative voltage.
- In other features, a pipelined analog to digital converter comprises a plurality of stages. A first one of the plurality of stages comprises the system. A flash analog to digital converter communicates with a last one of the plurality of stages. An alignment and digital error correction circuit communicates with the plurality of stages and the flash analog to digital converter.
- Further areas of applicability of the present disclosure will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the disclosure.
- The present disclosure will become more fully understood from the detailed description and the accompanying drawings, wherein:
-
FIG. 1 is a functional block diagram of a pipelined ADC according to the present disclosure; -
FIG. 2 is a functional block diagram of one of the stages of the pipelined ADC according to the present disclosure; -
FIG. 3 is a functional block diagram of a switched capacitor amplifier circuit according to the present disclosure; -
FIGS. 4A and 4B are functional block diagrams and electrical schematics of the switched capacitor amplifier circuit ofFIG. 3 in a sampling phase; -
FIG. 5 is a functional block diagram and electrical schematic of the switched capacitor amplifier circuit ofFIG. 4A in an amplifying phase; -
FIG. 6 is a functional block diagram and electrical schematic of an exemplary implementation of a comparator circuit; -
FIG. 7 is a functional block diagram and electrical schematic of an exemplary overdrive detect circuit; -
FIG. 8 is a graph illustrating residue voltage as a function of input voltage for an exemplary implementation; and -
FIG. 9 is a chart relating input voltages, MDAC and OD codes, residue voltages, switching, and output codes for the exemplary implementation ofFIG. 8 . - The following description is merely exemplary in nature and is in no way intended to limit the disclosure, its application, or uses. For purposes of clarity, the same reference numbers will be used in the drawings to identify similar elements. As used herein, the phrase at least one of A, B, and C should be construed to mean a logical (A or B or C), using a non-exclusive logical or. It should be understood that steps within a method may be executed in different order without altering the principles of the present disclosure.
- A clamping circuit according to the present disclosure provides improved overdrive recovery even when overdrive conditions occur frequently. The quick overdrive recovery tends to enhance system performance. The switch capacitor amplifier circuit with clamping according to the present disclosure will be described below in conjunction with a multiplying digital to analog converter (MDAC) stage of a pipelined analog to digital converter (ADC). However, skilled artisans will appreciate that the switched capacitor amplifier circuit according to the present disclosure may be used in other circuits.
- Referring now to
FIGS. 1 and 2 , a pipelined analog to digital converter (ADC) 10 includes multiple stages 12-1, 12-2, 12-3, . . . , and 12-N (collectively stages 12). Each of thestages 12 is used to determine subsets of the bits. The last stage 12-N communicates with aflash ADC 14. Thestages 12 and theflash ADC 14 communicate with an alignment and digitalerror correction circuit 16, which corrects for latency caused by the delays of thestages 12. Thepipelined ADC 10 converts the analog input signal to a T bit output, where T and N are integers and T>N. For example only, T may be set equal to 12 and N may be set equal to 4 in some implementations. - Each of the
stages 12 includes a sample andhold circuit 24, aflash ADC 26, a digital to analog converter (DAC) 30, asumming circuit 32 and anamplifier circuit 34. As will be described further below, the sample and holdcircuit 24, theDAC 30, thesumming circuit 32 and theamplifier circuit 34 in thestages 12 may be implemented by a MDAC with a switched capacitor amplifier circuit according to the present disclosure. - The analog input Vin is sampled and held by the sample and hold
circuit 24. Theflash ADC 26 quantizes the analog input Vin to M bits, where M may be set equal to T/N. For example only, M may be set equal to 3 in some implementations. The output of theflash ADC 26 is then fed to the DAC 30 (which may be accurate to about T bits). The analog output is subtracted from the input at the summingcircuit 32. - The residue voltage is amplified by the
amplifier circuit 34 and fed to the stage 12-2. Theamplifier 34 may have a gain. For example only, the gain may be set to 2 in some implementations. The gained-up residue voltage continues to another stage of thepipeline ADC 10, which provides M bits per stage. Theflash ADC 14 may resolve some of the LSB bits. For example only, theflash ADC 14 may resolve 4 LSBs in some implementations. - Because the M bits from each
stage 12 are determined at different points in time, all of the bits corresponding to the same sample of Vin are time-aligned by shift registers before being fed to time alignment and digitalerror correction circuit 16. When one of thestages 12 finishes processing a sample, determining the bits, and passing the residue voltage to the next stage, thestage 12 can start processing the next sample received from the sample and holdcircuit 24 of thestage 12. This pipelining action improves throughput. - Referring now to
FIG. 3 , a switchedcapacitor amplifier circuit 110 with clamping according to the present disclosure is shown. The switchedcapacitor amplifier circuit 110 includes sampling capacitors Cs. For example only, the number of sampling capacitors may be set equal to 2*2M. In the example inFIG. 3 , M=2 and 8 capacitors are provided, including Cs1p, Cs2p, Cs3p and Cs4p and Cs1m, Cs2m, Cs3m and Cs4m. The switchedcapacitor amplifier 110 further includes switches SW1p, SW2p, SW3p and SW4p that selectively switch terminals of the capacitors Cs1p, Cs2p, Cs3p and Cs4p to a positive or negative terminal of a reference voltage Vref or a positive terminal of an input voltage Vin. The switchedcapacitor amplifier 110 further includes switches SW1m, SW2m, SW3m and SW4m that selectively switch the capacitors Cs1m, Cs2m, Cs3m and Cs4m to the positive or negative terminal of the reference voltage Vref or a negative terminal of the input voltage Vin. - The switched
capacitor amplifier 110 further includes a multiplying DAC (MDAC)comparator circuit 120 that receives the input voltage Vin and that generates an MDAC code signal and switch control signals for switching the switches SW1m, SW2m, SW3m and SW4m and SW1p, SW2p, SW3p and SW4p. The switchedcapacitor amplifier 110 further includes an overdrive OD detectcircuit 130 that receives the input voltage Vin and that generates an OD code signal and switch control signals for clamping switches SWCLPin and SWCLPout. - The switched
capacitor amplifier 110 further includes an operational transconductance amplifier (OTA) 140. TheOTA 140 includes a non-inverting input that communicates with second terminals of the capacitors Cs1p, Cs2p, Cs3p and Cs4p and an inverting input that communicates with second terminals of the capacitors Cs1m, Cs2m, Cs3m and Cs4m. Feedback capacitor Cfp communicates with the non-inverting input and output of theOTA 140. Feedback capacitor Cfn communicates with the inverting input and output of theOTA 140. Input clamping switch SWCLPin communicates with the non-inverting and inverting inputs of theOTA 140. Output clamping switch SWCLPout communicates with the non-inverting and inverting outputs of theOTA 140. - When the
stages 12 are overdriven, it may take several clock cycles for theOTA 140 to provide the nominal level of accuracy. In some applications, this may result in system performance degradation. This problem is sometimes addressed by clamping the input signal before it is applied to the ADC in thestage 12. This type of clamping may impose penalties of power consumption and/or signal distortion. A clamping circuit according to the present disclosure provides overdrive recovery (the next clock cycle gain phase provides full accuracy) without distorting the signal and with minimal power dissipation. - Referring now to
FIG. 4A , the switchedcapacitor amplifier circuit 110 is shown in a sample phase. The switch control signals from theMDAC comparator circuit 120 connect first terminals of the capacitors Cs1p, Cs2p, Cs3p and Cs4p to a positive terminal of the input signal V. The switch control signals from theMDAC comparator circuit 120 connect the first terminals of the capacitors Cs1m, Cs2m, Cs3m and Cs4m to a negative terminal of the input signal Vin. The overdrive detectcircuit 130 generates the switch control signals to position the switches SWCLPin and SWCLPout in closed positions, which effectively shorts the OTA 140 (omitted fromFIG. 4A ). - During the sampling phase of the clock cycle, the input signal Vin is sampled. The input signal Vin is applied across the input sampling capacitors Cs1p, Cs2p, Cs3p and Cs4p and Cs1m, Cs2m, Cs3m and Cs4m while the feedback capacitors Cfp and Cfn are reset to zero. At the same time as the sampling phase, the
MDAC comparator circuit 120 makes a quantizer decision. The overdrive detectcircuit 130 detects whether or not the signal exceeds overdrive thresholds. - During the sampling phase, the
OTA 140 may be active as the amplifier for a subsequent MDAC stage. In other words, theOTA 140 may be switched and shared with another stage. For example only inFIG. 4B , switches S1 to S4 can be used to switch theOTA 140 between thestages 12 that are operating in opposite phases. As can be appreciated, other components fromFIG. 4A are omitted for purposes of clarity. - Following sampling of the input signal Vin, the MDAC is placed in the amplify phase as shown in
FIG. 5 . The switchedcapacitor amplifier circuit 110 is shown in an amplifying phase. The switch control signals from theMDAC comparator circuit 120 position the switches SW1p, SW2p, SW3p, SW4p, SW1m, SW2m, SW3m and SW4m based on the quantizer decision to connect the first terminals of the capacitors Cs1p, Cs2p, Cs3p, Cs4p, Cs1m, Cs2m, Cs3m and Cs4m to the positive or negative terminal of the reference voltage Vref. The clamping switches SWCLPin and SWCLPout may be opened or closed as will be described below. - In the amplifying phase, the capacitors Cs1m, Cs2m, Cs3m and Cs4m and Cs1p, Cs2p, Cs3p and Cs4p are connected to reference potentials in a configuration that is determined by the quantizer decision. If an overdrive condition is detected, the clamp switches SWCLPin and SWCLPout are closed. If the overdrive condition is not detected when the input signal Vin was sampled, the clamp switches SWCLPin and SWCLPout remain open and charge is transferred to satisfy forcing potentials (Vref+/−) and OTA gain, which results in a residue output VOUT. This is the normal operation that occurs when the overdrive detection and clamping are not performed. In other implementations, the clamp switches SWCLPin and SWCLPout are used when clamping is needed and additional switches (not shown) may be placed in parallel with SWCLPin and SWCLPout. The additional switches may be low impedance switches that are closed during the sampling phase (instead of the clamp switches SWCLPin and SWCLPout) and opened otherwise. Additional switches may also be used to reset the capacitors Cfp and Cfm.
- If an overdrive condition is detected when the input signal Vin is sampled, the switches SWCLPin and SWCLPout are closed. This forces the differential input and output of the
OTA 140 to zero. Charge on the sampling capacitors is redistributed to satisfy the forcing potentials (Vref+/−) and the zero volt (low impedance) condition at the inputs of theOTA 140. Depending on whether the overdrive polarity was positive or negative, the charge on the capacitors Cs1p, Cs2p, Cs3p and Cs4p and Cs1m, Cs2m, Cs3m and Cs4m and the OTA operating point will be exactly the same as if a positive or negative full scale signal was sampled. Here the clamp is shunting the feedback. - The clamp switch SWCLPout forces the output to zero since random offsets in the
OTA 140 would otherwise force the output into saturation since the feedback loop is broken. Since theOTA 140 is maintained at a valid operating point, it does not need to recover from an overdrive condition and will provide the nominal gain accuracy on the next clock cycle. The switches SWCLPin and SWCLPout can be very small and have negligible effect on normal operation. - The
overdrive detection circuit 130 also provides a digital output which allows the alignment digitalerror correction circuit 16 to properly clamp the digital output. Since the switch SWCLPout forces the MDAC output to zero, the composite output at the end of the pipeline has a digital value near full scale. Due to analog offsets, the digital output might be less than full scale, which would appear as a valid signal. The OD code output by theoverdrive detection circuit 130 is used to force digital clamping to full scale. - Referring now to
FIG. 6 , an exemplary implementation of theMDAC comparator circuit 120 is shown for a 1.0V, 12-bit ADC. TheMDAC comparator circuit 120 includes comparators 150-1, 150-2, 150-3 and 150-4 (collectively comparators 150) that receive the input voltage Vin and reference voltages. For the comparators 150-1 and 150-2, the input voltage Vin is connected to the inverting input and the non-inverting inputs receive −0.75V and −0.25V, respectively. For the comparators 150-3 and 150-4, the input voltage Vin is connected to the non-inverting input and the inverting inputs receive +0.25V and +0.75V, respectively. Outputs of the comparators 150 are input to adecoder 152, which provides a 2-bit output 154 that will be described further inFIGS. 8 and 9 . - Referring now to
FIG. 7 , an exemplary implementation of the overdrive detectcircuit 130 is shown. The overdrive detectcircuit 130 includes comparators 156-1 and 156-2 that receive the input voltage Vin at inverting and non-inverting inputs, respectively. The comparators 156-1 and 156-2 receive reference voltages of −1.25V and +1.25V at non-inverting and inverting inputs, respectively. Outputs of the comparators 156 are input to alogic gate 170 such as an OR gate. Anoutput 172 of thelogic gate 170 generates the OD code. - Referring now to
FIGS. 8 and 9 , additional operating details for the exemplary implementation are shown. InFIG. 8 , residue voltage is shown as a function of input voltage. InFIG. 9 , input voltage ranges, MDAC and OD codes, capacitor switching details, residue voltages and output codes are shown. - As can be appreciated, the
stage 12 with clamping may be the first stage of the pipelined analog to digital converter. Thesubsequent stages 12 do not need to have clamp circuits since the overdrive voltage will be handled by the first stage. - The broad teachings of the disclosure can be implemented in a variety of forms. Therefore, while this disclosure includes particular examples, the true scope of the disclosure should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification, and the following claims.
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US12/722,814 US8018274B1 (en) | 2010-03-12 | 2010-03-12 | Switched capacitor amplifier circuit with clamping |
CN201110060610.7A CN102195579B (en) | 2010-03-12 | 2011-03-14 | A kind of switched capacitor amplifier circuit with clamper |
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US12/722,814 US8018274B1 (en) | 2010-03-12 | 2010-03-12 | Switched capacitor amplifier circuit with clamping |
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US8493259B2 (en) * | 2011-12-06 | 2013-07-23 | Taiwan Semiconductor Manufacturing Co., Ltd. | Pipeline analog-to-digital converter |
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US20170373649A1 (en) * | 2016-06-27 | 2017-12-28 | Stichting Imec Nederland | Methods and Devices for Ramping a Switched Capacitor Power Amplifier |
US10574255B2 (en) * | 2017-11-06 | 2020-02-25 | Imec Vzw | Multiplying digital-to-analog conversion circuit |
WO2023022299A1 (en) * | 2021-08-17 | 2023-02-23 | 한국과학기술원 | Circuit and method for sensing bit-line multi-level voltage |
US20230095511A1 (en) * | 2021-09-21 | 2023-03-30 | Raytheon Company | High-energy suppression for infrared imagers or other imaging devices |
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US8493259B2 (en) * | 2011-12-06 | 2013-07-23 | Taiwan Semiconductor Manufacturing Co., Ltd. | Pipeline analog-to-digital converter |
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CN102195579A (en) | 2011-09-21 |
CN102195579B (en) | 2016-03-30 |
US8018274B1 (en) | 2011-09-13 |
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