# Patent

## Title
Offset cancellation for sampled-data circuits

## Abstract

A comparator based circuit with effective offset cancellation includes first and second amplifiers and an offset capacitor operatively connected to the first and second amplifiers. An offset voltage source generates an offset voltage. A first switch connects the offset voltage source to ground during a first time period. The first amplifier generates an output voltage in response to the first switch connecting the offset voltage source to ground during the first time period. A second switch connects the offset capacitor to ground during a second time period. The first switch disconnects the offset voltage source from ground during a third time period, and the second switch disconnects the offset capacitor from ground during the third time period.

## Background
This application is a divisional of and claims the priority benefit, under 35 U.S.C. §120 to U.S. Pat. No. 8,373,489, titled “OFFSET CANCELLATION FOR SAMPLED-DATA CIRCUITS,” listing inventor Hae-Seung Lee, filed on Oct. 21, 2010, which application is a divisional of and claims the priority benefit, under 35 U.S.C. §120 to U.S. Pat. No. 7,843,233, which application claims priority, under 35 U.S.C. §119(e), from U.S. Provisional Patent Application, Ser. No. 60/743,601, filed on Mar. 21, 2006, all of which are herein incorporated by reference.


BACKGROUND
A. Technical Field
The present invention relates generally to reference circuits for sampled-data circuits.
B. Background of the Invention
Most sample-data analog circuits such as switched-capacitor filters, analog-to-digital converters, and delta-sigma modulators require operational amplifiers to process the signal. Consider a switched-capacitor integrator example shown in FIG. 2. First, the switches S11 and S13 are closed so that the input voltage vin is sampled on the sampling capacitor CS1. Next, the switches S11 and S13 are opened and S12 and S14 are closed. This operation transfers the charge in the sampling capacitor CS1 to the integrating capacitor C11. The output voltage, vout, of a first integrator 1100 is typically sampled by another sample-data circuit, for example, another switched-capacitor integrator. In the circuit shown in FIG. 2, the circuit consisting of switches S21, S22, S23, S24, and a second sampling capacitor CS2 comprise a part of the second switched-capacitor integrator. The output voltage, vout, of the first integrator 10 is sampled on the second sampling capacitor CS2 by closing switches S21 and S23.
An example of a timing diagram is shown in FIG. 3. The clock signal has two non-overlapping phases φ1 and φ2. The phase φ1 is applied to switches S11, S13, S21, and S23, and phase φ2 is applied to switches S12, S14, S22, and S24. With this timing, the circuit performs non-inverting discrete integration with full clock delay. The waveforms at the output of the integrator, vout, and at the virtual ground node 100, v1, are also shown in FIG. 3. Different clock phasing arrangements yield different responses from the integrator. For example, if φ1 is applied to switches S11, S13, S22, and S24, and phase φ1 is applied to switches S12, S14, S21, and S23, the circuit performs non-inverting integration with half-clock delay.
For an accurate integration of the input signal, v1 must be driven as close to ground as possible. In order to accomplish this, the operational amplifier must provide sufficient open-loop gain and low noise. In addition, for fast operation, the operational amplifier 10 of FIG. 2 must settle fast.
In FIG. 3, the voltage v1 is shown to settle back to ground after a disturbance when the sampling capacitor CS1 is switched to Node 100 by closing S12 and S14. In addition to high open-loop gain and fast settling time, operational amplifiers must provide large output swing for high dynamic range. As the technology scales, it becomes increasingly difficult to achieve these characteristics from operational amplifiers. The primary factors that make the operational amplifier design difficult are low power supply voltages and low device gain.
As noted above, accurate output voltage can be obtained if Node 100 in FIG. 2 is maintained precisely at ground. However, in sample-data circuits, the only point of time accurate output voltage is required is at the instant the output voltage is sampled by another sampling circuit. Thus, it is not necessary to maintain the voltage at Node 100 at ground all the time.
Zero-crossing detectors can be applied in other switched-capacitor circuits such as algorithmic and pipeline analog-to-digital converters, delta-sigma converters, and amplifiers. These applications often require constant voltage sources, referred to as reference voltages.
Therefore, it is desirable to provide zero-crossing detectors in algorithmic analog-to-digital converters, pipeline analog-to-digital converters, delta-sigma converters, and amplifiers, which apply voltage sources, such as reference voltages, in zero-crossing detector based circuits in a manner that reduces the power consumption required in such voltage sources without degrading noise performance or speed of zero-crossing based circuits.
SUMMARY OF THE INVENTION
One aspect of the present invention is a comparator with effective offset cancellation. The comparator includes a first amplifier having an input terminal; an offset capacitor operatively connected to the first amplifier; a second amplifier, operatively connected to the offset capacitor; a first switch operatively connected to the input terminal; a second switch operatively connected between the offset capacitor and a first predetermined voltage; and a control circuit to control operations of the first and second switches. The first switch, in response to the control circuit, connects the input terminal to a second predetermined voltage during a time period. The second switch, in response to the control circuit, connects the offset capacitor to the first predetermined voltage during the time period. The time period is substantially shorter than a time constant associated with the offset capacitor.
Another aspect of the present invention is a comparator with effective offset cancellation. The comparator includes a first amplifier having an input terminal; a first offset capacitor operatively connected to the first amplifier; a second amplifier, operatively connected to the offset capacitor; a first switch operatively connected to the input terminal; a second switch operatively connected between the offset capacitor and a first predetermined voltage; a control circuit to control operations of the first and second switches; and a noise averaging circuit operatively connected to the first offset capacitor. The first switch, in response to the control circuit, connects the input terminal to a second predetermined voltage during a time period. The second switch, in response to the control circuit, connects the offset capacitor to the first predetermined voltage during the time period.
Another aspect of the present invention is a comparator based circuit with effective offset cancellation. The comparator based circuit includes a comparator to compare an input voltage with another voltage; a switched-capacitor network operatively connected to the comparator; an offset capacitor operatively connected to an input of the comparator; a preset voltage source, operatively connected to the offset capacitor, to charge the offset capacitor to a predetermined voltage; a waveform generator circuit, operatively connected to the offset capacitor, to apply a predetermined voltage waveform on the offset capacitor; and a sampling switch operatively connected to the offset capacitor. The sampling switch turns OFF when the comparator determines that a voltage at the input of the comparator crosses a predetermined level.
Another aspect of the present invention is a zero-crossing detector based circuit with effective offset cancellation. The zero-crossing detector based circuit includes a zero-crossing detector with a first input and a second input; a switched-capacitor network operatively connected to the zero-crossing detector; an offset capacitor operatively connected to the first input of the zero-crossing detector; a preset voltage source, operatively connected to the offset capacitor, to charge the offset capacitor to a predetermined voltage; a waveform generator circuit, operatively connected to the offset capacitor, to apply a predetermined voltage waveform on the offset capacitor; and a sampling switch operatively connected to the offset capacitor. The sampling switch turns OFF when the zero-crossing detector determines that a voltage at the first input crosses a ground potential.
Another aspect of the present invention is a level-crossing detector based circuit with effective offset cancellation. The level-crossing detector based circuit includes a level-crossing detector with a first input and a second input; a switched-capacitor network operatively connected to the level-crossing detector; an offset capacitor operatively connected to the first input of the level-crossing detector; a preset voltage source, operatively connected to the offset capacitor, to charge the offset capacitor to a predetermined voltage; a waveform generator circuit, operatively connected to the offset capacitor, to apply a predetermined voltage waveform on the offset capacitor; and a sampling switch operatively connected to the offset capacitor. The sampling switch turns OFF when the level-crossing detector determines that a voltage at the first input crosses a ground potential.
Another aspect of the present invention is a method of effectively cancelling offset in a zero-crossing detector based circuit. The method charges an offset capacitor to a predetermined voltage; applies a predetermined voltage waveform on the offset capacitor; determines if a voltage at an input of a zero-crossing detector has crossed a predetermined level; holds the voltage on the offset capacitor when it is determined that the voltage at the input of the zero-crossing detector has crossed the predetermined level; and cancels an offset of the zero-crossing detector using the voltage held on the offset capacitor when it is determined that the voltage at the input of the zero-crossing detector has crossed the predetermined level.
Another aspect of the present invention is a method of effectively cancelling offset in a level-crossing detector based circuit. The method charges an offset capacitor to a predetermined voltage; applies a predetermined voltage waveform on the offset capacitor; determines if a voltage at an input of a level-crossing detector has crossed a predetermined level; holds the voltage on the offset capacitor when it is determined that the voltage at the input of the level-crossing detector has crossed the predetermined level; and cancels an offset of the level-crossing detector using the voltage held on the offset capacitor.
Another aspect of the present invention is a method of effectively cancelling offset in comparator based circuit. The method charges an offset capacitor to a predetermined voltage; applies a predetermined voltage waveform on the offset capacitor; determines if a voltage at an input of a comparator has crossed a predetermined level; holds the voltage on the offset capacitor when it is determined that the voltage at the input of the comparator has crossed the predetermined level; and cancels an offset of the comparator using the voltage held on the offset capacitor.



BRIEF DESCRIPTION OF THE DRAWINGS
Reference will be made to embodiments of the invention, examples of which may be illustrated in the accompanying figures. These figures are intended to be illustrative, not limiting. Although the invention is generally described in the context of these embodiments, it should be understood that it is not intended to limit the scope of the invention to these particular embodiments.
FIG. 1 illustrates a zero-crossing detector.
FIG. 2 illustrates a switched-capacitor integrator.
FIG. 3 illustrates a timing diagram for the switched-capacitor integrator of FIG. 2.
FIG. 4 illustrates a non-inverting integrator according to the concepts of the present invention.
FIG. 5 illustrates a timing diagram for the non-inverting integrator of FIG. 4.
FIG. 6 illustrates a non-inverting integrator with a waveform generator being a current source according to the concepts of the present invention.
FIG. 7 illustrates another non-inverting integrator according to the concepts of the present invention.
FIG. 8 illustrates a timing diagram for the non-inverting integrator of FIG. 7.
FIG. 9 illustrates another non-inverting integrator according to the concepts of the present invention.
FIG. 10 illustrates another non-inverting integrator according to the concepts of the present invention.
FIG. 11 illustrates a timing diagram for the non-inverting integrator of FIG. 10.
FIG. 12 illustrates another non-inverting integrator according to the concepts of the present invention.
FIG. 13 illustrates another non-inverting integrator according to the concepts of the present invention.
FIG. 14 illustrates a timing diagram for the non-inverting integrator of FIG. 13.
FIG. 15 illustrates a closed-loop offset cancellation circuit.
FIG. 16 illustrates an open-loop offset cancellation circuit.
FIG. 17 illustrates a timing diagram for the open-loop offset cancellation circuit of FIG. 16.
FIG. 18 illustrates another timing diagram for the open-loop offset cancellation circuit of FIG. 16.
FIG. 19 illustrates another open-loop offset cancellation circuit.
FIG. 20 illustrates a third timing diagram for the open-loop offset cancellation circuit of FIG. 19.
FIG. 21 illustrates a zero-crossing detector circuit with offset cancellation.
FIG. 22 illustrates another zero-crossing detector circuit with offset cancellation.
FIG. 23 illustrates a third zero-crossing detector circuit with offset cancellation.
FIG. 24 illustrates a fourth zero-crossing detector circuit with offset cancellation.
FIG. 25 illustrates a fully-differential implementation of the fourth zero-crossing detector circuit of FIG. 24.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention will be described in connection with preferred embodiments; however, it will be understood that there is no intent to limit the present invention to the embodiments described herein. On the contrary, the intent is to cover all alternatives, modifications, and equivalents as may be included within the spirit and scope of the present invention, as defined by the appended claims.
For a general understanding of the present invention, reference is made to the drawings. In the drawings, like reference have been used throughout to designate identical or equivalent elements. It is also noted that the various drawings illustrating the present invention may not have been drawn to scale and that certain regions may have been purposely drawn disproportionately so that the features and concepts of the present invention could be properly illustrated.
It is noted that, in the various Figures, the earth symbol indicates the system's common-mode voltage. For example, in a system with 2.5 V and −2.5 V power supplies, the system's common-mode voltage may be at ground. In a system with a single 2.5 power supply, the system's common-mode voltage may be at 1.25 V.
As noted above, accurate output voltage can be obtained if Node 100 in FIG. 2 is maintained precisely at ground. However, in sampled-data circuits, the only point of time accurate output voltage is required is at the instant the output voltage is sampled by another sampling circuit. Thus, it is not necessary to maintain the voltage at Node 100 at ground all the time.
FIG. 4 illustrates a non-inverting integrator according to the concepts of the present invention. More specifically, as an example, a non-inverting integrator with half-clock delay is illustrated in FIG. 4.
As illustrated in FIG. 4, a clock phase φ1 is applied to switches S11, S13, S22, and S24, and another phase φ2 is applied to switches S12, S14, and S21. A zero crossing detector 30 is used to detect the point of time at which Node 100 crosses ground. The switch S23 is controlled by the output of the zero crossing detector 30. The output of the zero crossing detector 30 is used to determine the time point to take the sample of the output voltage vout. A waveform generator 20 generates a voltage waveform as the output voltage vout in such way the voltage at Node 100 crosses zero if the charge in capacitors CS1 and Cu is within a normal operating range.
In the timing diagram shown in FIG. 5, the waveform generated by the waveform generator 20 is shown as a ramp. When v1, the voltage at Node 100, crosses zero at time t1, the output vzc of the zero crossing detector 30 goes low, turning the switch S23 OFF. At that instant, the output voltage vout is sampled on CS2.
Since v1 is very close to zero when the sample of v2 is taken, an accurate output voltage is sampled on CS2. A similar operation repeats during the next clock cycle, and the sample of the output voltage is taken at time t2.
It is noted that the zero crossing detector 30 may optionally have an overflow detection feature that determines when the charge in capacitors CS1 and Cl1 is outside the normal range of operation. It can be implemented by a logic circuit that makes the output vzc of the zero-crossing detector 30 to go low when φ2 goes low. In the event v1 fails to cross zero, the sample is taken on the falling edge of φ2. At the same time, the logic circuit produces a flag indicating overflow.
In the embodiment described above and in the various embodiments described below, a zero crossing detector is utilized in lieu of a comparator. Typically, a comparator is designed to compare two arbitrary input voltages. A comparator may be implemented as cascaded amplifiers, a regenerative latch, or a combination of both. A comparator may be used to detect a zero voltage level or a predetermined voltage level crossing.
It is noted that the input waveform of the various described embodiments is not arbitrary, but deterministic and repetitive. Thus, the various described embodiments determine the instant the zero voltage level or the predetermined voltage level is crossed than relative amplitudes of the input signals. For such a deterministic input, a zero crossing detector is more efficient.
An example of a zero-crossing detector for the detection of a positive-going input signal is shown in FIG. 1. Initially, node 1 and node 2 are precharged to vDD and ground, respectively. The ramp input voltage vin is applied according to the zero crossing circuit. At the time the input node crosses the threshold, node 1 is discharged rapidly, and node 2 is pulled up to vDD. Since the zero crossing detector in FIG. 1 is a dynamic circuit, there is no DC power consumption, allowing extremely low power and fast operation. For the detection of zero-crossing of a negative-going signal, a complementary circuit with a PMOS input transistor can be utilized.
As illustrated in FIG. 6, the non-inverting integrator includes a waveform generator which is a current source 200. As illustrated in FIG. 6, a clock phase φ1 is applied to switches S11, S13, S22, and S24, and another phase φ2 is applied to switches S12, S14, and S21. A zero crossing detector 30 is used to detect the point of time at which Node 100 crosses ground. The switch S23 is controlled by the output of the zero crossing detector 30. The output of the zero crossing detector 30 is used to determine the time point to take the sample of the output voltage vout.
The current source 200 charges the capacitors CS2 and the series connected CS1 and Cl1, generating a ramp. At the start of φ2, the output is briefly shorted to a known voltage VNEG, the value of which is chosen to ensure the voltage v1 at Node 100 crosses zero with signals in the normal operating range.
As illustrated in FIG. 7, the non-inverting integrator includes a waveform generator 20 that produces, preferably, a plurality of segments in the waveform with varying rate of change of the output voltage. The first segment may be controlled so as to have the highest rate of change, with subsequent segments having progressively lower rate of change. The detection of zero crossing by the zero crossing detector 30 causes the waveform to advance to the next segment. An output signal vzc2 of the zero crossing detector 30 remains high until the zero crossing is detected in the last segment of the waveform.
One clock cycle of the timing diagram is shown in FIG. 8. At the start of φ2, the waveform generator 20 produces an up ramp. The voltage v1 is shown to cross zero at time t1. One output, vzc1, of the zero crossing detector 30 changes its state after a finite delay td1.
The delay td1 represents finite delay of a typical zero crossing detector 30. This change of state advances the waveform to the next segment.
Due to the delay td1 of the zero crossing detector 30, the voltage V1 overshoots by a small amount above ground. The second segment of the waveform generator is a down ramp to permit another zero crossing at time t2. After a second delay td2, the output vzc2 of the zero crossing detector 30 goes low, causing the switch S23 to turn OFF, locking the sample of the output voltage vout.
The delay td2 of the second zero crossing is not necessarily the same as the delay associated with the first zero crossing td1. The delay td2 contributes a small overshoot to the sampled output voltage. The effect of the overshoot can be shown to be constant offset in the sampled charge. In most sample-data circuits, such constant offset is of little issue.
The zero crossing detector 30 preferably becomes more accurate in detecting the zero crossing as the segments of the waveform advances. The first detection being a coarse detection, it doesn't have to be very accurate. Therefore, the detection can be made faster with less accuracy. The last zero crossing detection in a given cycle determines the accuracy of the output voltage. For this reason, the last zero crossing detection must be the most accurate.
The accuracy, speed, and the power consumption can be appropriately traded among progressive zero crossing detections for the optimum overall performance. For example, the first detection is made less accurately and noisier but is made faster (shorter delay) and lower power. The last detection is made more accurately and quieter while consuming more power or being slower (longer delay).
An example of a two-segment waveform generator constructed of two current sources (210 and 220) is shown in FIG. 9. As illustrated in FIG. 9, a clock phase φ1 is applied to switches S11, S13, S22, and S24, and another phase φ2 is applied to switches S12, S14, and S21. A zero crossing detector 30 is used to detect the point of time at which Node 100 crosses ground. The switch S23 is controlled by the output of the zero crossing detector 30. The output of the zero crossing detector 30 is used to determine the time point to take the sample of the output voltage vout.
Current sources 210 and 220 charge the capacitors CS2 and the series connected CS1 and Cl1 generating two segments of a ramp waveform. At the start of φ2, the output is briefly shorted to a known voltage vNEG, the value of which is chosen to ensure the voltage v1 crosses zero with signals in the normal operating range. During the first segment, the current source 210 is directed to the output, while during the second segment, the current source 220 is directed to the output, generating two different slopes of ramp.
As illustrated in FIG. 10, the non-inverting integrator includes a level crossing detector 300 having plurality of thresholds. As illustrated in FIG. 10, a clock phase φ1 is applied to switches S11, S13, S22, and S24, and another phase φ2 is applied to switches S12, S14, and S21. A level crossing detector 300 is used to detect the point of time at which Node 100 crosses one of plurality of predetermined levels as discussed below. The switch S23 is controlled by the output of the level crossing detector 300. The output of the level crossing detector 300 is used to determine the time point to take the sample of the output voltage vout.
The thresholds are predetermined voltage levels. The thresholds of the level crossing detector 300 can be adjusted to minimize overshoot.
For example, the threshold for the first detection may be made negative by a slightly smaller amount than the expected overshoot in the first segment. This minimizes the ramp-down time in the second segment. Also, the threshold for the second segment may be made more positive by the amount of the overshoot in the second segment in order to cancel the effect of the overshoot. Alternatively, the threshold for the first segment may be made more negative than the expected overshoot during the first segment. This permits the second segment to be a positive ramp rather than a negative ramp as shown in FIG. 11.
It is advantageous to make the detection during the last segment to be the most accurate detection. The accuracy of the detection during the last segment is made higher than during other segments. This can be achieved by making the delay longer or making the power consumption higher during the last segment.
As illustrated in FIG. 12, the non-inverting integrator includes a level crossing detector having two zero-crossing detectors, Zero Crossing Detector 1 (310) and Zero Crossing Detector 2 (320). As illustrated in FIG. 12, a clock phase φ1 is applied to switches S11, S13, S22, and S24, and another phase φ2 is applied to switches S12, S14, and S21. Zero Crossing Detector 1 (310) and Zero Crossing Detector 2 (320) are used to detect the point of time at which Node 100 crosses one of plurality of predetermined levels as discussed below. The switch S23 is controlled by the output of the Zero Crossing Detector 2 (320). The output of the Zero Crossing Detector 2 (320) is used to determine the time point to take the sample of the output voltage vout.
The thresholds of the Zero Crossing Detector 1 (310) and Zero Crossing Detector 2 (320) are selected to minimize overshoot. For example, the threshold for Zero Crossing Detector 1 (310) may be made negative by a slightly smaller amount than the expected overshoot in the first segment. This minimizes the ramp-down time in the second segment. Also, the threshold for Zero Crossing Detector 2 (320) may be made more positive by the amount of the overshoot in the second segment in order to cancel the effect of the overshoot. Alternatively, the threshold for Zero Crossing Detector 1 (310) may be made more negative than the expected overshoot during the first segment. This permits Zero Crossing Detector 2 (320) to be a positive ramp rather than a negative ramp.
In other words, Zero Crossing Detector 1 (310) makes a coarse detection, whereas Zero Crossing Detector 2 (320) makes a fine detection. Thus, it is advantageous to make Zero Crossing Detector 2 (320) to have a higher accuracy.
As illustrated in FIG. 13, the non-inverting integrator includes a level crossing detector having two zero-crossing detectors, Zero Crossing Detector 1 (310) and Zero Crossing Detector 2 (320). As illustrated in FIG. 13, a clock phase φ1 is applied to switches S11, S13, S22, and S24, and another phase φ2 is applied to switches S12, S14, and S21. Zero Crossing Detector 1 (310) and Zero Crossing Detector 2 (320) are used to detect the point of time at which Node 100 crosses one of plurality of predetermined levels as discussed below. The switch S23 is controlled by the output of the Zero Crossing Detector 2 (320). The output of the Zero Crossing Detector 2 (320) is used to determine the time point to take the sample of the output voltage vout.
Both detectors, Zero Crossing Detector 1 (310) and Zero Crossing Detector 2 (320), have nominally zero thresholds. The detection thresholds are determined by voltages vtr1 and vtr2 applied to the inputs of Zero Crossing Detector 1 (310) and Zero Crossing Detector 2 (320), respectively. Zero Crossing Detector 1 (310) makes a coarse detection, whereas Zero Crossing Detector 2 (320) makes a fine detection. Thus, it is advantageous to make Zero Crossing Detector 2 (320) to have a higher accuracy.
It is noted that the above-described embodiment may operate as a self-timed system. In this configuration, Rather than supplying constant frequency clock phases φ1 and φ2, the clock phases are derived from the outputs of Zero Crossing Detector 1 (310) and Zero Crossing Detector 2 (320). FIG. 14 illustrates a self-timed operation.
As illustrated in FIG. 14, the end of the phase φ2 is defined by the output of the detection during the last segment. The beginning of the clock phase φ1 is defined by a short delay, such as logic delays, after the end of φ2. The short delay is generally necessary to ensure non-overlapping clock phases. The end of the clock phase φ1 is determined by the zero crossing detection of the previous stage or the following stage in the similar manner.
It is noted that zero crossing detector based circuits require substantially less power consumption compared with operational amplifier based circuits at a given sampling rate and signal-to-noise ratio because the noise bandwidth of a zero crossing detector is much lower than that of an operational amplifier at a given sampling rate. Zero crossing detectors can be applied in other switched-capacitor circuits such as algorithmic and pipeline analog-to-digital converters, delta-sigma converters, and amplifiers.
In applications where high precision is required, the effects of the offset voltage due to device mismatch must be mitigated. In switched-capacitor circuits, offset cancellation techniques are often employed to reduce the offset voltage.
An example of a circuit with closed-loop offset cancellation is illustrated in FIG. 15. As illustrated in FIG. 15, during the sampling phase, the input voltage VIN is applied to the sampling capacitor CS, and the switch S1 is closed. The voltage sampled on CS is VIN−VOS. The voltage sampled on CS cancels the effect of offset voltage due to device mismatch. It is noted that a control or logic circuit (not shown) is utilized to control the operations of switches S1 and S2.
In zero-crossing detector based circuits, similar closed-loop offset cancellation results are feasible by closing the loop around the first stage of the zero-crossing detector. The noise bandwidth during the closed-loop offset sampling is comparable to that in operational amplifier based circuits. The high noise bandwidth of the closed-loop offset sampling adds significant amount of noise and at least partially negates the low noise advantage of zero-crossing detector based circuits.
An open-loop offset cancellation is illustrated in FIG. 16. As illustrated in FIG. 16, open-loop offset cancellation can be used with voltage comparators. The timing diagram for the open-loop offset cancellation is shown in FIG. 17. During the offset sampling phase, TOS, the input of the first amplifier A1 is connected to ground through switch S1. The input of the second amplifier A2 is also connected to ground by closing the switch S2. The output voltage of the first amplifier A1 settles to −a1VOS, where a1 is the voltage gain of the amplifier A1.
The settling time constant τ is equal to RoC where Ro is the Thevenin output resistance of the first amplifier A1, and C is the parallel combination of parasitic capacitance Cp1 and COS. The switch S2 is then opened, whereby −a1VOS is sampled and held across the offset storage capacitor COFF. During the normal operation phase, switch S1 connects the input of first amplifier A1 to the voltage VIN. The effective input voltage to first amplifier A1 is VIN−VOS due to the effect of the offset voltage VOS.
The output voltage of first amplifier A1 is then a1(VIN−VOS). The input voltage to second amplifier A2 is a1(VIN−VOS)−(a1VOS)=a1VIN. Thus, the effect of the offset voltage of the first amplifier A1 is removed.
For accurate offset cancellation, the offset cancellation phase TOS must be at least several times longer than the time constant τ. This requires wide bandwidth in the first amplifier A1 that corresponds to high noise. Although it is possible to employ the open-loop offset cancellation in zero-crossing detectors, as with the closed-loop offset cancellation, the high noise bandwidth of the open-loop offset sampling adds significant amount of noise and partially negates the low noise advantage of zero-crossing detector based circuits.
As noted above, it is desirable to provide offset cancellation in zero crossing detectors without substantially increasing the noise of zero-crossing detectors. An example of offset cancellation in zero crossing detectors can be realized using the same circuit structure, as illustrated in FIG. 15, but with different timing and a different capacitor value for COFF. An example of a timing diagram for offset cancellation in zero crossing detectors is shown in FIG. 18.
As illustrated in FIG. 18, the offset cancellation phase TOFF is divided into two sub-phases, T1 and T2. During the phase T1, switch S1 is connected to ground while switch S2 is still open. The settling time constant τ1 during this phase is approximately Ro(Cp1+Cp2), assuming COFF>>Cp2. Since both Cp1 and Cp2 are small parasitic capacitances, τ1 is short. Therefore, the output voltage V01 quickly settles to −a1VOS.
During the phase T2, switch S2 is closed with switch S1 still connected to ground. The offset storage capacitor COFF is made substantially larger such that the settling time constant τ2=RoC during T2 is comparable to or longer than T2. Such a long time constant reduces the noise bandwidth during the phase T2. The bandwidth of noise sampled in COFF is determined by τ2, thereby providing low noise.
Although τ2 is long, the accuracy of offset cancellation is not affected because the voltage across COFF reaches −a1VOS, and there is no change in the voltage across COFF during T2. In practice, small disturbance in the voltage occurs at the time switch S2 is closed due to capacitive coupling. The disturbance is constant and also reduced by a1 when referred to the input, and hence poses little concern in most systems.
After the offset cancellation is complete, the switch S1 is connected to VIN, and switch S2 is open for normal operation as a zero-crossing detector or a comparator.
In another zero-crossing detector according to the second embodiment is shown to have two amplifier stages, first stage amplifier A1, and the second stage amplifier S2 as shown in FIG. 19. The second stage amplifier A2 may be either a linear amplifier or a regenerative latch. The switch S3 and the capacitor COFF2 perform averaging of sampled noise to lower the noise. The timing diagram is shown in FIG. 20.
During the offset cancellation phase TOFF, the input of the first amplifier A1 is connected to ground by throwing the switch S1 to the upper position. The input of the second amplifier A2 is also connected to ground by closing the switch S2. The output voltage of the first amplifier A1 settles to −a1VOS. The capacitor COFF1 is made sufficiently small so that the output settles to an accurate value during TOFF. At the end of TOFF, the switch S2 is then opened, whereby −a1VOS is sampled and held across the capacitor COFF1. During the normal operation phase following the offset cancellation phase, S1 is thrown the input voltage VIN, and S3 is closed.
After a few clock cycles of operation, the voltage across COFF2 converges to the voltage sampled on COFF1, −a1VOS. Since COFF2 is larger than COFF1, the sampled noise is averaged and reduced by a factor of (1+COFF2/COFF1)1/2. The effective input voltage to A1 is VIN−VOS due to the effect of the offset voltage VOS. The output voltage of A1 is then a1(VIN−VOS). The input voltage to A2 is a1(VIN−VOS)−(−a1VOS)=a1VIN. Thus, the effect of the offset voltage of the first amplifier A1 is removed, but the sampled noise is substantially lower.
Another example of offset cancellation is illustrated in FIG. 21. For clarity, a zero-crossing detector based integrator similar to that illustrated in FIG. 4 is shown. The current source I produces ramp waveforms, functioning as a waveform generator. The current source IOFF and the capacitor COFF sample the offset voltage of the zero-crossing detector ZCD1 to cancel its effect. During the offset cancellation phase, one input IN1 is grounded by closing switches S13 and S14. The switch SFB is left open in order not to disturb the charge on the integrating capacitor CI1. Switch SOFF2 is closed, and switch SOFF1 is briefly closed to precharge the capacitor COFF to a voltage VOFF. Next, switch SOFF1 is opened, and IOFF is integrated on COFF. The value of IOFF is chosen in such way that the voltage at node IN2 ramps down during offset cancellation at approximately the same rate as the node voltage IN2 ramps up during the normal operation.
When the zero-crossing detector ZCD1 detects the crossing of the voltage V2 at the input IN2 of ground potential, the switch SOFF2 is turned OFF. Shortly after, the current source IOFF is turned OFF, sampling the voltage on COFF. The voltage sampled on COFF is shown to be substantially equal in magnitude and opposite in sign to the offset VOS of the zero-crossing detector ZCD1.
During the subsequent operation of the circuit as an integrator, switch SOFF1 is turned OFF, and switch SOFF2 is left ON. The voltage at the input IN2 at the ZCD1 is thus maintained at −VOS, thus, the effect of offset VOS in the zero-crossing detector ZCD1 is cancelled during integration operation. It is noted that a control or logic circuit (not shown) is utilized to control the operations of the various switches.
Another example of offset cancellation is illustrated in FIG. 22. This embodiment is similar to the embodiment of FIG. 21, except two capacitors, COFF1 and COFF2, store the offset voltage. Capacitor COFF2 is larger than capacitor COFF1. For clarity, a zero-crossing detector based integrator similar to that illustrated in FIG. 4 is shown.
The current source IOFF and the capacitor COFF1 sample the offset voltage of the zero-crossing detector ZCD1 to cancel its effect. The value of IOFF is chosen in such way that the voltage at node IN2 ramps down during offset cancellation at approximately the same rate as the node voltage IN2 ramps up during the normal operation.
During the offset cancellation phase, one input IN1 is grounded by closing switches S13 and S14. The switch SFB is left open in order not to disturb the charge on the integrating capacitor CI1. Switch SOFF2 is closed, switch SOFF3 is opened, and switch SOFF1 is briefly closed to precharge the capacitor COFF1 to a voltage VOFF. Next, switch SOFF1 is opened, and IOFF is integrated on capacitor COFF1.
When the zero-crossing detector ZCD1 detects the crossing of the voltage V2 at the input IN2 at ground potential, the switch SOFF2 is turned OFF, sampling the voltage on capacitor COFF1. The current source IOFF is then turned OFF, and switches SOFF2 and SOFF3 are closed causing the change in capacitors COFF1 and SOFF2 to be averaged. This effectively averages sampled noise, and reduces the noise. The voltage stored on capacitors COFF1 and COFF2 is shown to be substantially equal in magnitude and opposite in sign to the offset VOS of the zero-crossing detector ZCD1.
During the subsequent operation of the circuit as an integrator, switch SOFF1 is turned OFF, and switches SOFF2 and SOFF3 are left ON. Alternatively, switches SOFF1 and SOFF2 are turned OFF, and switch SOFF3 is left ON. The voltage at the input IN2 of the ZCD1 is thus maintained at −VOS, thus, the effect of offset VOS in the zero-crossing detector ZCD1 is cancelled during integration operation. It is noted that a control or logic circuit (not shown) is utilized to control the operations of the various switches.
Another example of offset cancellation is illustrated in FIG. 23. This embodiment is similar to the embodiment of FIG. 21, except the offset sampling capacitor COFF is placed in series with the input IN1, and the current source IOFF is chosen such that the ramp rate at node IN1 is approximately the same during the offset cancellation and the normal operation. It is noted that a control or logic circuit (not shown) is utilized to control the operations of the various switches.
The current source IOFF and the capacitor COFF1 sample the offset voltage of the zero-crossing detector ZCD1 to cancel its effect. The value of IOFF is chosen in such way that the voltage at node IN1 ramps down during offset cancellation at approximately the same rate as the node voltage IN1 ramps up during the normal operation.
During the offset cancellation phase, a switch SFB is left open in order not to disturb the charge on the integrating capacitor CI1, switch SOFF2 is closed, and switch SOFF1 is briefly closed to precharge the capacitor COFF to a voltage VOFF. Next, switch SOFF1 is opened, and IOFF is integrated on COFF.
When the zero-crossing detector ZCD1 detects the crossing of the voltage V2 at the input IN1 of ground, the switch SOFF2 is turned OFF. Shortly after, the current source IOFF is turned OFF. The voltage sampled on capacitor COFF is shown to be substantially equal in magnitude and opposite in sign to the offset of the zero-crossing detector ZCD1. Therefore, the effect of offset in the zero-crossing detector ZCD1 is cancelled during subsequent operation.
Another example of offset cancellation is illustrated in FIG. 24. This embodiment is similar in operation to the embodiment of FIG. 22, except the offset is first sampled on a first offset sampling capacitor COFF1 and averaged with a second offset sampling capacitor COFF2 in a similar fashion to the embodiment illustrated in FIG. 22. The current source IOFF and the capacitor COFF1 sample the offset voltage of the zero-crossing detector ZCD1 to cancel its effect. The value of IOFF is chosen in such way that the voltage at node IN1 ramps down during offset cancellation at approximately the same rate as the node voltage IN1 ramps up during the normal operation.
During the offset cancellation phase, a switch SFB is left open in order not to disturb the charge on the integrating capacitor CI1, switch SOFF2 is closed, and switch SOFF1 is briefly closed to precharge the capacitor COFF1 to a voltage VOFF. Next, switch SOFF1 is opened, and IOFF is integrated on COFF1.
When the zero-crossing detector ZCD1 detects the crossing of the voltage V2 at the input IN1 of ground, the switch SOFF1 is turned OFF. Shortly after, the current source IOFF is turned OFF. The voltage sampled on capacitor COFF1 is shown to be substantially equal in magnitude and opposite in sign to the offset of the zero- crossing detector ZCD1. Switch SOFF3 is then closed, connecting COFF1 and COFF2 in parallel. The charge in COFF1 and COFF2 is redistributed, averaging the sampled noise. After a few cycles of offset cancellation, the voltage stored on capacitors COFF1 and COFF2 is shown to be substantially equal in magnitude and opposite in sign to the offset Vas of the zero-crossing detector ZCD1. Therefore, the effect of offset in the zero-crossing detector ZCD1 is cancelled during subsequent operation.
Although the concepts of the present invention have been illustrated and described in connection with single-ended embodiments, the concepts of the present invention are also applicable to fully-differential configurations or fully-differential implementations of these single-ended embodiments.
For example, a fully-differential implementation of the embodiment illustrated in FIG. 24 is illustrated in FIG. 25.
The current sources IOFFp, IOFFn and the capacitors COFFp, and COFFp sample the offset voltage of the zero-crossing detector ZCD1 differentially to cancel its effect. The values of IOFFp and IOFFn are chosen in such way that the difference between voltages at node IN1p and INT1n ramps down during offset cancellation at approximately the same rate as the difference between voltages at node INT1p and INT1n ramps up during the normal operation.
During the offset cancellation phase, switches SFBp and SFBn are left open in order not to disturb the charge on the integrating capacitors CI1p and CI1n, switches SOFF2p and SOFF2n are closed, and switch switches SOFF1p and SOFF1n are briefly closed to precharge the capacitors COFFp and COFFn to voltages COFFp and VOFFn, respectively. Next, switched SOFF1 is opened, and IOFF is integrated on COFF.
When the zero-crossing detector ZCD1 detects the zero or level crossing of the difference between voltages V2p and V2n at the input IN1p and IN1n, respectively, the switches SOFF2p and SOFF2n are turned OFF. Shortly after, the current sources IOFFp and IOFFn are is turned OFF. The difference between voltages sampled on capacitor COFFp and COFFn is shown to be substantially equal in magnitude and opposite in sign to the offset of the zero-crossing detector ZCD1. Therefore, the effect of offset in the zero-crossing detector ZCD1 is cancelled during subsequent operation.
Although the concepts of the present invention have been illustrated and described in connection with zero-crossing detector based circuits, the concepts of the present invention are also applicable to comparator based circuits. While various examples and embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that the spirit and scope of the present invention are not limited to the specific description and drawings herein, but extend to various modifications and changes.

## Claims

1. A comparator with effective offset cancellation, the comparator comprising:
a first amplifier comprising an input terminal;
an offset capacitor coupled to the first amplifier;
a second amplifier coupled to the offset capacitor;
a first switch coupled to the input terminal;
a second switch coupled between the offset capacitor and a first predetermined voltage; and
a control circuit to control operations of the first and second switches, the first switch, in response to the control circuit, couples the input terminal to a second predetermined voltage for a period of time, the second switch, in response to the control circuit, couples the offset capacitor to the first predetermined voltage for the period of time, the period of time being substantially shorter than a time constant associated with the offset capacitor.

2. The comparator as claimed in claim 1, wherein the first predetermined voltage is a system common-mode voltage.

3. The comparator as claimed in claim 2, wherein the system common-mode voltage is a ground potential.

4. The comparator as claimed in claim 1, wherein the second predetermined voltage is a system common-mode voltage.

5. The comparator as claimed in claim 4, wherein the system common-mode voltage is a ground potential.

6. A comparator with effective offset cancellation, comprising:
a first amplifier comprising an input terminal;
a first offset capacitor coupled to the first amplifier;
a second amplifier coupled to the offset capacitor;
a first switch coupled to the input terminal;
a second switch coupled between the offset capacitor and a first predetermined voltage;
a control circuit to control operations of the first and second switches; and
a noise averaging circuit coupled to the first offset capacitor, the first switch, in response to the control circuit, couples the input terminal to a second predetermined voltage for a period of time, the second switch, in response to the control circuit, couples the offset capacitor to the first predetermined voltage for the period of time.

7. The comparator as claimed in claim 6, wherein the first predetermined voltage is a system common-mode voltage.

8. The comparator as claimed in claim 7, wherein the system common-mode voltage is a ground potential.

9. The comparator as claimed in claim 6, wherein the second predetermined voltage is a system common-mode voltage.

10. The comparator as claimed in claim 9, wherein the system common-mode voltage is a ground potential.

11. The comparator as claimed in claim 6, wherein the noise averaging circuit includes a second offset capacitor.