Patent Description:
In many electronics applications, an analog input signal is converted to a digital output signal (e.g., for further digital signal processing). For instance, in precision measurement systems, electronics are provided with one or more sensors to make measurements, and these sensors can generate an analog signal. The analog signal can then be provided to an analog-to-digital converter (ADC) circuit as input to generate a digital output signal for further processing. In another instance, in a mobile device receiver, an antenna can generate an analog signal based on the electromagnetic waves carrying information/signals in the air. The analog signal generated by the antenna can then be provided as input to an ADC to generate a digital output signal for further processing.

<CIT> discloses a correlated double sampling analog-to-digital converter including techniques to cancel kT/C sampling noise and residue amplifier sampling noise while also reducing power consumption in a pipelined analog-to-digital converter circuit.

Various analog-to-digital converter (ADC) topologies exist, including delta-sigma, pipelined converters, flash, and successive approximation register (SAR) converters. Noise sources in an ADC circuit can include kT/C sampling noise from a capacitor DAC circuit, noise coupling on to sampling capacitors from digital circuits and amplifier thermal noise. In conventional ADC architectures, kT/C sampling noise is inversely proportional to the size of the sampling capacitors; larger sampling capacitors can produce less noise. However, larger sampling capacitors can be difficult to drive and can physically occupy significant die area.

By using various techniques of this disclosure, the effect of these noise sources can be greatly reduced, allowing both lower noise conversion and smaller sampling capacitors, which can reduce the die area and reduce the power consumption of the ADC.

The invention is defined by the appended independent method claim <NUM> and the independent apparatus claim <NUM>. Preferred embodiments are defined by the appended dependent claims <NUM>-<NUM> and <NUM>-<NUM>.

Various analog-to-digital converter (ADC) topologies exist, including delta-sigma, pipelined converters, flash, and successive approximation register (SAR) converters. Noise sources in an ADC circuit can include kT/C noise of a sampling capacitor, noise coupling on to sampling capacitors from digital circuits and amplifier noise. Also, charge injection from mismatch in sample switches can cause offsets. The kT/C sampling noise is inversely proportional to the size of the sampling capacitors; larger sampling capacitors can produce less noise. However, larger sampling capacitors can be difficult to drive and can physically occupy significant die area.

By using various techniques of this disclosure, these various noise sources can be largely canceled or reduced. As a result, the size of the sampling capacitors can be greatly reduced, while still achieving significantly improved noise performance and power efficiency for the overall converter. In the following description, various noise sources, including kT/C noise, digital noise and offsets are lumped together and described as a noise voltage "n".

<FIG> is a simplified schematic diagram of an example of an ADC circuit that can implement various techniques of this disclosure. The ADC circuit <NUM> can include a plurality of switches S1-S5, an ADC input <NUM>, a DAC circuit <NUM>, a DAC logic circuit <NUM>, a control circuit <NUM>, a converter circuit <NUM> including an amplifier circuit <NUM>, e.g., with a gain G ideally greater than <NUM>, such as <NUM>, and a quantizer circuit <NUM>, e.g., a comparator circuit, coupled to an output of the converter circuit <NUM>. As seen in <FIG>, the quantizer circuit <NUM> can be coupled to the output of the converter circuit <NUM> via a latch circuit <NUM> that can be controlled by a strobe signal ("STRB"). As shown in <FIG>, in some example configurations, the converter circuit can include more than one output. Among other things, the control circuit <NUM> can control the operation of the switches S1-S5, for example.

An ADC input <NUM> can receive an analog input signal VIN and an estimated value of the analog input signal VIN_ESTIMATE. The analog input signal VIN_ESTIMATE is the value of VIN that is estimated to occur at a sampling instance. The ADC circuit <NUM> can include a first capacitor circuit C1 coupled to both the ADC input <NUM> and the converter circuit <NUM>, including the amplifier circuit <NUM>. The quantizer circuit <NUM>, coupled to the output of the amplifier circuit <NUM>, can further include a second capacitor circuit C2. Optionally, and as described in more detail below, the ADC circuit <NUM> can include or be coupled to an ADC sub-circuit <NUM> that is coupled to analog input signal Vin. The ADC sub-circuit <NUM> can perform a coarse conversion of the analog input signal VIN and preload the DAC circuit <NUM> with the results of the conversion.

In some example configurations, the circuit <NUM> can include a buffer amplifier (not depicted) coupled to the ADC input to buffer the analog input signal prior to sampling the analog input signal. In some configurations, the quantizer circuit <NUM> can include a comparator circuit, which can be configured to perform an auto-zero operation to reduce or cancel an input offset voltage.

Optionally, in some example implementations, one or more impedance elements, such as resistors and/or capacitors, can be coupled to the amplifier circuit <NUM> in a negative feedback configuration. For example, using an optional feedback capacitor <NUM>, the output of the amplifier circuit <NUM> can be coupled to the inverting input of the amplifier circuit <NUM> to create a virtual ground at the input of the amplifier circuit <NUM>.

An output of the converter circuit <NUM> can be coupled to the DAC logic circuit <NUM>. In some example implementations, the DAC logic circuit <NUM> can implement a successive approximation binary search or other search algorithm. The DAC logic circuit <NUM> can be coupled to an input of the DAC circuit <NUM> and can generate outputs "d1" to apply to the DAC circuit <NUM> during the successive approximation search process. In this manner, the ADC circuit <NUM> includes a feedback configuration in which the DAC circuit <NUM> can be responsive to an output of the converter circuit <NUM>. The DAC circuit <NUM> can adjust the first capacitor circuit based on an output of the converter circuit <NUM>.

In the following example operation of the ADC circuit <NUM>, the DAC logic circuit <NUM> implements a successive approximation search. The optional ADC <NUM> is also not used. The operation of the ADC circuit <NUM> is shown and described below with respect to Table <NUM>:.

Table <NUM> (above) depicts six rows (labeled "T1" through "T6") that represent time at the start of each phase of operation. Starting from the left-hand side of the table, columns <NUM>-<NUM> (labeled "S1" through "S5") depict the states of the switches S1-S5, respectively. Columns <NUM> and <NUM> (labeled "A1[V]" and "A2[V]") depict corresponding voltages at nodes A1 and A2 in <FIG>. Column <NUM> depicts voltages at node Vcap in <FIG>, and Column <NUM> on the right-hand side depicts the operation that occurs at the beginning or during each phase.

For purposes of explanation, the switches in this disclosure are assumed to be N-type enhancement mode field-effect transistors (FETs), although other types of transistors can be used and are considered within the scope of this disclosure. The operation of the ADC circuit of <FIG> will now be described with reference to Table <NUM> where a logic high signal ("<NUM>") corresponds to a closed switch and a logic low signal ("<NUM>") corresponds to an open switch.

At time T1 (row <NUM>), the control circuit <NUM> controls the switches S2, S4, and S5 to close and the switches S1 and S3 to open. As part of acquisition phase <NUM>, an estimated value of the analog input signal ("VIN_ESTIMATE") is fed, via switch S2, to the left-hand side plate of the first capacitor circuit C1. The right-hand side plate of C1 is shorted by the switch S4 to a bias or ground and the voltages at node A1 (the output of the amplifier circuit <NUM>) and node A2 (the input of the quantizer circuit <NUM>) are 0V.

In some example implementations, the signal VIN_ESTIMATE can be derived from a previous conversion result. In other example implementations, the signal VIN_ESTIMATE can be derived from an ADC sub-circuit, such as ADC1 in <FIG>, or from a separate ADC circuit, or from VIN_itself as shown in <FIG>.

At time T2 (row <NUM>), the control circuit <NUM> controls the switches S2 and S4 to open. Opening the switch S4 decouples the first capacitor circuit C1 from a bias voltage, e.g., ground or some other voltage, and samples the estimated voltage VIN_ESTIMATE of the analog input signal on to C1. Opening the switch S4 results in a sampled noise voltage on C1 ("n" in Table <NUM>) which is amplified or transferred to the output of the amplifier <NUM>, e.g., node A1. More particularly, the voltage at node A1 takes on the value G*n, where G is the gain of the amplifier circuit <NUM>. The voltage at the node A2 is 0V because the closed switch S5 results in the negative input of the quantizer circuit <NUM> being at virtual ground.

At time T3 (row <NUM>), the control circuit <NUM> controls the switch S1 to close. As part of acquisition phase <NUM>, an analog input signal VIN can be received, via the ADC input, and transferred by switch S1 to the node Vcap (the left-hand side plate of the first capacitor circuit C1).

The sampled noise component "n" and a difference between the analog input signal VIN and the sampled value of the estimated voltage VIN_ESTIMATE of the analog input signal can be received and amplified by the amplifier circuit <NUM>, resulting in a voltage of G*(VIN -Ve +n), where Ve equals VIN_ESTIMATE, at the node A1. As indicated above, in some configurations, at least one impedance element can be coupled to the amplifier circuit <NUM> in a negative-feedback configuration. With the capacitor <NUM> placed between the input and the output of the amplifier, as shown, the amplifier is in an inverting configuration and hence G can be negative. The voltage at the node A2 is 0V because the closed switch S5 results in the negative input of the quantizer circuit <NUM> still being at virtual ground.

If the switch S1 remains closed and if the analog input signal VIN moves, a moving difference is generated between the analog input signal VIN and the estimated value VIN_ESTIMATE, which is gained up by the amplifier circuit <NUM> and transferred to the second capacitor circuit C2 as a moving difference.

At time T4 (row <NUM>), the control circuit <NUM> controls the switches S1 and S5 to open (so all the switches S1-S5 are open) to sample the voltage G*(VIN - Ve +n) on node A1 on to capacitor C2 at the 'sampling instance' of the ADC while the first capacitor circuit is receiving the analog input signal. The voltage of the analog input signal VIN and also on Vcap at the sampling instance is 'Vin at sampling instance'. Opening the switch S5 samples a representation of a combination of the noise voltage and a difference signal that is generated between the estimated value of the analog input signal and the analog input signal. Again, the analog input signal VIN_ESTIMATE is the value of VIN that is estimated to occur at a sampling instance.

At time T5 (row <NUM>), the control circuit <NUM> controls the switch S3 to close while the remaining switches S1, S2, S4, and S5 remain open. In this phase of operation, the voltage at node A2 is servoed to 0V over a number of quantizing steps of a successive approximation (SAR) search, of the analog input signal at the sampling instance. The DAC logic circuit <NUM> controls the DAC circuit <NUM> to adjust the first capacitor circuit C1 based on outputs of the converter circuit to perform a conversion. Using the DAC logic circuit <NUM> and the DAC circuit <NUM>, the value of the Vcap node is adjusted. Each adjustment to the Vcap node causes a corresponding adjustment to the output voltage of amplifier <NUM> whose voltage is sensed by quantizer <NUM>. Multiple adjustments to the Vcap node are made until the voltage at the right-hand side of the second capacitor circuit C2 is at or close to zero.

Looking at voltages on nodes A1, A2 and Vcap at T6, their voltages have returned to the values they were at in phase T4. At the very beginning of phase T4, Vcap node was driven by Vin and the voltage Vin was captured on Vcap. At time T6, Vcap is driven by DAC <NUM>. For A2 to be at zero volts during both time T4 and T6, A1 and most importantly Vcap must also be at the same voltage. In other words, the DAC output voltage must be equal to Vin (at the sampling instance) and the conversion result is represented by the digital value at the input of DAC <NUM>. In real implementations, A2 may not be at exactly 0V at T6, due to the finite resolution of the DAC. Note that the noise voltage "n" and the value of VIN_ESTIMATE do not come in to play and so do not affect the accuracy of the conversion result.

It is also notable that the voltage stored across capacitor C2 at the instant of switch S5 opening (at time T4) is preserved throughout the conversion process. The converter <NUM> uses the sampled voltage across C2 to generate its digital outputs during the successive approximation process. While the converter uses C2, the actual voltage across C2 doesn't affect the digital values that drive the DAC <NUM> during the successive approximation. That digital value, and the corresponding DAC output voltage must equal Vin at the sampling instance at the end of conversion.

In a SAR operation, an analog input voltage VIN_can be sampled and held using a sampling circuit and a differential output voltage of a digital-to-analog converter (DAC) circuit can be compared to the sampled and held voltage using a comparator circuit. The bit values of the DAC circuit can be adjusted based on the output of the comparator circuit. SAR operation is known to those of ordinary skill in the art and, for purposes of conciseness, will not be described in detail in this disclosure. An example SAR ADC is described in commonly assigned <CIT> and titled "DIFFERENTIAL INPUT SUCCESSIVE APPROXIMATION ANALOG TO DIGITAL CONVERTER WITH COMMON MODE REJECTION,".

In optional phases of the conversion process, noise can be filtered, e.g., using a dynamic filter, to reduce a noise bandwidth of the difference signal (generated between the estimated value of the analog input signal and for example, the analog input signal at the sampling instance) amplified by amplifier circuit <NUM>. As shown in <FIG>, the ADC circuit <NUM> can include a dynamic filter that can include a resistive component R1, and a bypass switch <NUM> to bypass the resistive component R1. Additionally, or alternatively, the amplifier circuit <NUM> can include a dynamic filter circuit configured to perform bandwidth reduction.

To reduce noise sampled onto the second capacitor circuit C2, e.g., thermal noise of the amplifier, the resistive component R1 can initially be bypassed when switch <NUM> is closed, which can allow a voltage to quickly settle onto the second capacitor circuit C2. Once the voltage has settled, the control circuit <NUM> can open switch <NUM> thereby placing the resistive component R1 in series with the second capacitor circuit C2, which can band limit the noise on C2. As a non-limiting example, the resistor R1 can be about <NUM> kiloohm and the capacitor C2 can be about <NUM> pF, resulting in a bandwidth of about <NUM> megahertz.

It should be noted that the ADC circuit <NUM> need not perform all of the conversion. Optionally, in some example implementations, a second ADC circuit <NUM> can perform a first portion of the conversion and the ADC circuit <NUM> can perform a second, remaining portion. For example, the second ADC circuit <NUM>, e.g., SAR, flash, sigma-delta, etc., can sample the analog input signal and provide the result to the DAC circuit <NUM>. At the instant when the switch S4 or the switch S5 opens, for example, the separate ADC circuit <NUM> can also sample the analog input signal. The second ADC circuit <NUM> can then proceed to perform a conversion. After this portion of the overall conversion result is obtained from the second ADC circuit <NUM>, the portion can be loaded onto the DAC circuit <NUM>, and the remaining part of the conversion result can be obtained by operating the successive approximation search.

A benefit of using a separate converter circuit is that it can be much faster than the main converter circuit because the separate converter circuit attempts to resolve only a few bits, with lower demands on accuracy. Speeding up the overall conversion has the benefit of needing to keep circuits powered up for shorter spans of time, thereby reducing power.

In another example implementation for slow moving inputs, previous conversion results can be loaded onto the DAC circuit <NUM>, and then some of the least significant bits (LSBs) bit trials can be retested to account for any change in the input from the previous sample to the present.

In some example configurations, the first capacitor circuit C1 can include a capacitor array having a capacitive digital-to-analog converter (DAC) circuit, such as shown in <FIG>.

<FIG> is a schematic diagram of a portion of <FIG> depicting a capacitive digital-to-analog converter (CDAC) circuit that can form a part of the ADC circuit <NUM>. In <FIG>, the first capacitor circuit C1, S1 and S2 and also DAC <NUM> of <FIG> have been replaced by a capacitor array forming a part of a capacitive digital-to-analog converter (DAC) circuit <NUM>. The CDAC circuit <NUM> can include capacitors C1-CN, a plurality of switches to couple one or more of the capacitors C1-CN to VIN, a plurality of switches to couple one or more of the capacitors C1-CN to VIN_ESTIMATE, and a plurality of switches to couple one or more of the capacitors C1-CN to a driver, each controlled by a digital input code D(h). When capacitors are being controlled by the digital input, the effective voltage VDAC can be considered as a weighted sum of the voltages on the driven nodes of the capacitors.

In some such configurations, when receiving the estimated value VIN_ESTIMATE of the analog input signal, at least one capacitor in the capacitor array can be charged to a voltage that is responsive to the analog input signal VIN. For example, the charging can include biasing at least one terminal of the capacitor array to a voltage that is equal to or linearly related to the analog input signal, such as by using a filter circuit.

In some example configurations, such as seen in <FIG>, a digital code D(h) can be applied to the CDAC circuit <NUM>. For example, the control circuit <NUM> of <FIG> can apply a digital dither code D(h) to at least part of the CDAC circuit <NUM>. In some example implementations, the DAC logic circuit <NUM> can apply a dither code during a sampling of the analog input signal VIN_ESTIMATE before the switch S4 has opened. In other example implementations, the control circuit <NUM> can apply a dither code after the sampling of the analog signal VIN_ESTIMATE, after the switch S4 has opened, but before a conversion.

As described above with respect to <FIG>, the analog input signal VIN_ sampled at the sampling instance includes the difference signal (where the difference signal is the difference between the estimated value of the analog input signal VIN_ESTIMATE, and the actual value of VIN at the sampling instance). The DAC logic circuit <NUM> can control the CDAC circuit <NUM> to determine a digital value representative of the combination of the estimated value of the analog input signal and the sampled value of the difference signal. For example, in some implementations, the DAC logic circuit <NUM> can use successive approximation to determine and apply a sequence of at least two digital codes to the capacitive digital-to-analog converter (DAC) circuit to determine the digital value. While it is true that the sampled value of the difference signal contains the noise voltage "n", the digital representation of the combination of the estimated value of the analog input signal and the sampled value of the difference signal, which is used to drive the DAC circuit, doesn't. The output voltage of the DAC circuit at the end of the conversion process must equal Vin at the sampling instance, including neither the noise voltage "n", nor the estimated value of the analog input signal VIN_ESTIMATE.

<FIG> is a simplified schematic diagram of another example of an ADC circuit that can implement various techniques of this disclosure. The ADC circuit <NUM> of <FIG> can include components similar to those shown in <FIG>, with like elements indicated by like reference numerals. In the ADC circuit <NUM> of <FIG>, the capacitor circuit for the most significant bits (MSBs) is represented by the capacitor circuit C1, e.g., a capacitor array, coupled to the DAC circuit <NUM> and referred to as an "MDAC". Capacitors in the MDAC can be shuffled to improve linearity.

In addition, the ADC circuit <NUM> can include a capacitor circuit for the least significant bits (LSBs) that is represented by the capacitor C3, e.g., a capacitor array, coupled to the DAC circuit <NUM> and referred to as an "LDAC". The LDAC circuit can be coupled to the input of the amplifier circuit <NUM>. Capacitors in the LDAC can be used to apply dither, e.g., dither d2 applied to the LDAC. In some example implementations, dither can be added to the MDAC. The LDAC and the MDAC can operate in combination to improve the linearity of ADC circuit <NUM>.

Advantageously, using various techniques of this disclosure, the capacitors of the MDAC and the LDAC do not have to be sized with respect to noise performance because no matter what noise is sampled, the noise is canceled. That is, large capacitors are not needed for the MDAC and the LDAC to achieve a high SNR. Thus, the size of the sampling capacitors can be greatly reduced, which can reduce the die area and reduce the power consumption of the ADC. In this manner, a lower noise level can be achieved using the same power or, for the same noise level, less power can be used.

<FIG> is a simplified schematic diagram of another example of an ADC circuit that can implement various techniques of this disclosure. The ADC circuit <NUM> can include a plurality of switches S1, S3-S5, an ADC input <NUM>, a DAC circuit <NUM> ("DAC1"), a DAC logic circuit <NUM>, a control circuit <NUM>, a converter circuit <NUM> including an amplifier circuit <NUM>, e.g., with a gain G ideally greater than <NUM>, such as <NUM>, and a quantizer circuit <NUM>, e.g., an analog-to-digital converter circuit ADC2. Among other things, the control circuit <NUM> can control the operation of the switches S1, S3-S5, for example. The ADC input <NUM> can receive an analog input signal VIN. The ADC circuit <NUM> can include a first capacitor circuit C1 coupled to both the ADC input <NUM>, the converter circuit <NUM> and an amplifier circuit <NUM>. The quantizer circuit <NUM>, coupled to the output of the amplifier circuit, can further include a second capacitor circuit C2 and a sampling switch S5.

In some example configurations, the ADC circuit <NUM> can include a buffer amplifier (not depicted) coupled to the ADC input to buffer the analog input signal prior to sampling the analog input signal. Optionally, in some example implementations, one or more impedance elements, such as resistors and/or capacitors, can be coupled to the amplifier circuit <NUM> in a negative feedback configuration. For example, using an optional feedback capacitor <NUM>, the output of the amplifier circuit <NUM> can be coupled to the inverting input of the amplifier circuit <NUM> to create a virtual ground at the input of the amplifier circuit <NUM>.

The output of the converter circuit, e.g. provided by the output of the quantizer circuit <NUM>, can be coupled to the DAC logic circuit <NUM>. The DAC logic circuit <NUM> can be coupled to an input of the DAC1 circuit <NUM> and can generate outputs "d1" to apply to the DAC1 circuit <NUM>. In this manner, the ADC circuit <NUM> includes a feedback configuration in which the DAC circuit <NUM> can be responsive to an output of the converter circuit <NUM>.

In this example configuration, the comparator <NUM> of <FIG> has been replaced with the quantizer circuit ADC2 <NUM> and also the switch S2 has been removed, as the value of VIN_ESTIMATE is captured in a different manner. Both modifications to <FIG> can be made on their own or in combination.

An example of the operation of the ADC circuit <NUM> is shown and described below with respect to Table <NUM>:.

Table <NUM> (above) depicts eight rows (labeled "T1" through "T8") that represent time at the start of each phase of operation. Starting from the left-hand side of the table, columns <NUM>-<NUM> (labeled "S1", "S3", "S4" and "S5") depict the states of the switches S1 and S3-S5, respectively. Column <NUM> (labeled "Vamp") depicts the voltage at node Vamp at the input of the quantizer circuit <NUM> ("ADC2") of <FIG>. Column <NUM> (labeled "Converter operation") depicts the operation of a corresponding ADC and/or DAC in the ADC circuit <NUM>. Column <NUM> depicts voltages at node Vcap in <FIG>, and Column <NUM> on the right-hand side depicts the operation that occurs at the beginning or during the phase.

At time T1 (row <NUM>), the control circuit <NUM> controls the switches S1, S4 and S5 to close and the switch S3 to open. In this acquisition phase, the analog input signal VIN is fed to the left-hand side plate of the first capacitor circuit C1. The right-hand side plate of C1 is shorted by the switch S4 to a bias voltage or ground and the voltage at node Vamp (the output of the amplifier circuit <NUM>) is 0V.

At time T2 (row <NUM>), the control circuit <NUM> controls the switches S4 to open. Opening the switch S4 decouples the first capacitor circuit C1 from a bias voltage, e.g., ground or some other voltage, and samples the value VIN on to C1. The value of Vin when S4 opens, in this example configuration, is an estimate of the value of VIN at the sample instance and is called VIN_ESTIMATE. Opening the switch S4 results in a noise voltage on the first capacitor circuit C1 ("n" in Table <NUM>) which is transferred to the output of the amplifier <NUM>, e.g., node Vamp. More particularly, the voltage at node Vamp is G*(n), where G is the gain of the amplifier circuit <NUM> and "n" is the noise voltage component generated from opening the switch S4.

During the phase T3 a noise voltage "n" and a difference between the analog input signal VIN and the sampled value of the estimated voltage VIN_ESTIMATE of the analog input signal can be received and amplified by the amplifier circuit <NUM>, resulting in a voltage of G*(VIN -Ve +n), where Ve equals VIN_ESTIMATE, at the node Vamp. As indicated above, in some configurations, at least one impedance element can be coupled to the amplifier circuit <NUM> in a negative-feedback configuration.

If the switch S1 remains closed and if the analog input signal VIN moves, a moving difference is generated between the analog input signal VIN and the estimated value VIN_ESTIMATE, which is gained up by the amplifier circuit <NUM> and transferred to the second capacitor circuit C2 as a difference. If the analog input signal VIN does not move, then the estimated value VIN_ESTIMATE equals the analog input signal VIN at the sampling instance, there is no voltage change at the output node Vamp of the amplifier circuit <NUM>, and only G*n is stored on the second capacitor circuit C2.

At time T4 (row <NUM>), the control circuit <NUM> controls the switch S5 (of ADC2 in <FIG>) to open to sample the voltage G*(VIN -Ve +n) on node Vamp on to capacitor C2 at the 'sampling instance' of the ADC while the first capacitor circuit C1 is receiving the analog input signal. Opening of the switch S5 samples a representation of the combination of the noise voltage and a difference signal that is generated between the estimated value of the analog input signal and the analog input signal.

The quantizer circuit ADC2 of <FIG> converts the sampled voltage and the digital result is DADC2_1. The voltage of the analog input signal VIN and also on the node Vcap at the sampling instance is 'VIN at sampling instance'.

To minimize gain error caused by the amplifier circuit <NUM>, it is desirable that the voltage on the node Vamp is close to zero. Using the techniques below, the output DADC2 of the ADC2 circuit <NUM> can modify the DAC1 circuit <NUM> that can, in turn, generate a smaller voltage on the node Vamp until that voltage is firstly within an input range of ADC2 circuit <NUM> and secondly close to zero (if it is required to reduce the impact of gain error in the amplifier circuit <NUM>). In this manner, the DAC logic circuit <NUM> can adjust the first capacitor circuit C1 based on an output of the converter circuit to perform a conversion of the analog input signal at the sampling instance.

At time T5 (row <NUM>), the control circuit <NUM> controls the switches S3 and S5 to close and the switch S1 to open. In this phase, the voltage at node Vamp is G*(VDAC1-<NUM>-Ve+n). The DAC logic circuit <NUM> loads the DAC1 circuit <NUM> to a mid-scale value VDAC1-<NUM>, for example. The DAC1 circuit <NUM> can generate the voltage VDAC1_1, which is the voltage at the node Vcap. In this phase S5 is closed and ADC2 <NUM> is in an acquire mode.

At time T6 (row <NUM>), the control circuit <NUM> controls the switch S5 to open and the ADC2 circuit <NUM> of <FIG> samples and converts the voltage on the node Vamp, or G*(VDAC1_1-Ve+n), with the digital result being DADC2_2.

At time T7 (row <NUM>), the control circuit <NUM> controls the switch S5 to close. To reduce the magnitude of the voltage at node Vamp, the DAC logic circuit <NUM> modifies the DAC1 circuit <NUM> input based on the output DADC2_2 of ADC2 circuit <NUM>. The ADC2 circuit <NUM> of <FIG> acquires the voltage on node Vamp, or G*(VDAC1_2 - Ve+n).

At time T8 (row <NUM>), the control circuit <NUM> controls the switch S5 to open and the ADC2 circuit <NUM> of <FIG> samples and converts the voltage on the node Vamp, or G*(VDAC1_2-Ve+n), with the digital result being DADC2_3. The voltage on the node Vcap is VDAC1_3. The operations described in times T6 and T7 can be repeated, e.g., using successive approximation, until the output DADC2 of the ADC2 circuit <NUM> is not overloaded and is ideally close to zero. The conversion of the signal VIN at the sampling instance can be calculated from.

ADC2 conversion results and the final DAC1 input value, as shown and derived below where N is the resolution of ADC2 and Vref is ADC2's reference voltage: <MAT> <MAT> <MAT> <MAT> <MAT> where <MAT>.

Note that both Ve and "n" are subtracted out in Equation (<NUM>) and so do not affect the accuracy of the conversion. In optional phases of the conversion process, noise from switches, the amplifier <NUM> or on the input Vin can be filtered, e.g., using a dynamic filter, to reduce a noise bandwidth of the difference signal (generated between the estimated value of the analog input signal and for example, the analog input signal at the sampling instance) amplified by amplifier circuit <NUM>. As shown in <FIG>, the ADC2 circuit <NUM> can include a dynamic filter that can include a resistive component R1, and a bypass switch <NUM> to bypass the resistive component R1, as described above with respect to <FIG>.

As mentioned above, in some example configurations, the converter circuit <NUM> can include more than one output. For example, the converter circuit <NUM> can optionally include a second quantizer circuit <NUM>, e.g., a comparator circuit or ADC circuit, that can provide a second output of the converter circuit <NUM>.

As shown in <FIG>, an input of the second quantizer circuit <NUM> can be coupled to the input of the amplifier circuit <NUM>, e.g., the node on the right-hand side plate of the first capacitor circuit C1. Alternatively, in some example configurations, an input of the second quantizer circuit <NUM> can be coupled to the output of the amplifier circuit <NUM>, e.g., the node Vamp.

The output of the second quantizer circuit <NUM> can provide a result to a second output of the converter circuit <NUM> which may be provided faster than can be provided by the first quantizer. As such, the converter circuit <NUM> can include at least a first output (from the ADC2 circuit) and a second output (from the second quantizer circuit <NUM>).

The second output of the converter circuit <NUM> (from the second quantizer circuit <NUM>) can be used, for example, for a portion, e.g., an initial portion, of the successive approximation search and used to adjust the DAC1 circuit <NUM> until Vamp is within the full-scale range of ADC2 <NUM>.

With ADC2 being a multi-bit quantizer, the least significant bits can be determined by ADC2 with fewer modifications to DAC1 than would be required if the quantizer input had to be driven to close to 0V, as is required in the circuit shown in <FIG>. As a result, the circuit shown in <FIG> can be expected to be faster and DAC1 can be have a resolution of two or more bits less than the resolution of the overall conversion result (the digital value).

<FIG> is a simplified schematic diagram of another example of an ADC circuit that can implement various techniques of this disclosure. The ADC circuit <NUM> can include a plurality of switches S1-S5, an ADC input <NUM>, a DAC circuit <NUM> ("DAC1"), a DAC logic circuit <NUM>, a control circuit <NUM>, a converter circuit <NUM> including an amplifier circuit <NUM>, e.g., with a gain G ideally greater than <NUM>, such as <NUM>, a quantizer circuit <NUM>, e.g., an analog-to-digital converter circuit ADC2, coupled to an output of the converter circuit <NUM>. Among other things, the control circuit <NUM> can control the operation of the switches S1-S5, for example.

In addition, the ADC circuit <NUM> of <FIG> can include an ADC circuit <NUM> ("ADC1") that receives an analog input voltage VIN and outputs a digital representation DADC1 that is fed to the DAC logic and control circuit <NUM>.

The ADC input <NUM> can receive an analog input signal VIN and an estimated value of the analog input signal VIN_ESTIMATE. The ADC circuit <NUM> can include a first capacitor circuit C1 coupled to both the ADC input <NUM> and an amplifier circuit <NUM>. The quantizer circuit <NUM>, coupled to the output of the amplifier circuit, can further include a second capacitor circuit C2 and a sampling switch S5.

In some example configurations, the circuit <NUM> can include a buffer amplifier (not depicted) coupled to the ADC input to buffer the analog input signal prior to sampling the analog input signal. Optionally, in some example implementations, one or more impedance elements, such as resistors and/or capacitors, can be coupled to the amplifier circuit <NUM> in a negative feedback configuration. For example, using an optional feedback capacitor <NUM>, the output of the amplifier circuit <NUM> can be coupled to the inverting input of the amplifier circuit <NUM> to create a virtual ground at the input of the amplifier circuit <NUM>.

An output of the converter circuit <NUM> can be coupled to the DAC logic circuit <NUM>. In some example implementations, the DAC logic circuit <NUM> can implement a successive approximation (SAR) search. The DAC logic circuit <NUM> can be coupled to an input of the DAC circuit <NUM> and can generate outputs "d1" to apply to the DAC circuit <NUM>, e.g., during a SAR process. In this manner, the ADC circuit <NUM> includes a feedback configuration in which the DAC circuit <NUM> can be responsive to an output of the converter circuit <NUM>.

The ADC circuit <NUM> can include an additional converter circuit ADC3 containing switches S6 and S7, and capacitors C3 and C4. An example of the operation of the ADC circuit <NUM> is shown and described below with respect to Table <NUM>. For the purposes of this example, we will ignore the converter circuit ADC3.

Table <NUM> (preceding) depicts eight rows (labeled "T1" through "T8") that represent time at the start of each phase of operation. Starting from the left-hand side of the table, columns <NUM>-<NUM> (labeled "S1" through "S5") depict the states of the switches S1-S5, respectively. Column <NUM> (labeled "Vamp") depicts the voltage at node Vamp at the input of the quantizer circuit <NUM> of <FIG>. Column <NUM> (labeled "Converter operation") depicts the operation of a corresponding ADC and/or DAC in the ADC circuit <NUM>. Column <NUM> depicts voltages at node Vcap in <FIG>, and Column <NUM> on the right-hand side depicts the operation that occurs at the beginning or during each phase.

At time T1 (row <NUM>), the control circuit <NUM> controls the switches S2, S4, and S5 to close and the switches S1 and S3 to open. In this acquisition phase, an estimated value of the analog input signal ("VIN_ESTIMATE") is fed, via switch S2, to the left-hand side plate of the first capacitor circuit C1. The right-hand side plate of C1 is shorted by the switch S4 to a bias voltage or ground and the voltage at node Vamp (the output of the amplifier circuit <NUM>) is 0V.

At time T2 (row <NUM>), the control circuit <NUM> controls the switches S2 and S4 to open. Opening the switch S4 decouples the first capacitor circuit C1 from a bias voltage, e.g., ground or some other voltage, and samples the estimated voltage VIN_ESTIMATE of the analog input signal on to C1. Opening the switch S4 results in a noise voltage on C1 ("n" in Table <NUM>) which is transferred to the output of the amplifier <NUM>, e.g., node Vamp. More particularly, the voltage at node Vamp is G*(n), where G is the gain of the amplifier circuit <NUM> and "n" is the noise voltage component generated from opening the switch S4.

At time T3 (row <NUM>), the control circuit <NUM> controls the switch S1 to close. In this acquisition phase, an analog input signal VIN can be received, via the ADC input, and transferred by switch S1 to the node Vcap (the left-hand side plate of the first capacitor circuit C1).

A voltage equivalent to the noise voltage component "n" and a difference between the analog input signal VIN and the sampled value of the estimated voltage VIN_ESTIMATE of the analog input signal can be received and amplified by the amplifier circuit <NUM>, resulting in a voltage of G*(VIN -Ve +n), where Ve equals VIN_ESTIMATE, at the node Vamp. As indicated above, in some configurations, at least one impedance element can be coupled to the amplifier circuit <NUM> in a negative-feedback configuration.

If the switch S1 remains closed and if the analog input signal VIN moves, a moving difference is generated between the analog input signal VIN and the estimated value VIN_ESTIMATE, which is gained up by the amplifier circuit <NUM> and transferred to the second capacitor circuit C2 as a difference.

At time T4 (row <NUM>), the control circuit <NUM> controls the switch S5 (of ADC2 <NUM> in <FIG>) to open to sample the voltage G*(VIN -Ve +n) on node Vamp on to capacitor C2 at the 'sampling instance' of the ADC while the first capacitor circuit is receiving the analog input signal VIN. Opening the switch S5 samples a representation of a combination of the noise voltage and a difference signal that is generated between the estimated value of the analog input signal and the analog input signal. The ADC2 circuit <NUM> of <FIG> converts the sampled voltage and the digital result is DADC2_1. The voltage of the analog input signal VIN and also on the node Vcap at the sampling instance is 'VIN at sampling instance'.

As seen in <FIG>, the ADC circuit <NUM> can include an ADC1 circuit <NUM> coupled to the DAC logic circuit <NUM>. The ADC1 circuit <NUM> can receive the analog input signal VIN and output a digital representation DADC1, which is loaded into the DAC1 circuit <NUM> (via the DAC logic circuit <NUM>). As a result, the voltage on node Vcap changes, and the voltage on node Vamp changes.

At time T5 (row <NUM>), the control circuit <NUM> controls the switches S3 and S5 to close and the switch S1 to open. In this phase, the voltage at node Vamp is G*(VDAC1_1-Ve+n). The DAC logic circuit <NUM> can load the DAC circuit <NUM> using the output DADC1 of the ADC1 circuit <NUM> and the DAC1 circuit <NUM> can generate an output voltage VDAC1_1, which is the voltage at the node Vcap.

To minimize or eliminate any gain error caused by the amplifier circuit <NUM>, it is desirable that the voltage on the node Vamp is close to zero. Using the techniques below, the output DADC2 of the ADC2 circuit <NUM> can modify the DAC1 circuit <NUM> that can, in turn, generate a smaller voltage on the node Vamp.

At time T7 (row <NUM>), the control circuit <NUM> controls the switch S5 to close. To reduce the magnitude of the voltage at node Vamp, the DAC logic circuit <NUM> modifies the DAC1 circuit <NUM> input based on the output DADC2_2 of ADC2, e.g., an output of the converter circuit <NUM>, to perform a conversion of the analog input signal at the sampling instance. The voltage on the node Vcap is VDAC1_2. The ADC2 circuit <NUM> of <FIG> acquires the voltage on node Vamp, or G*(VDAC1_2 - Ve+n).

At time T8 (row <NUM>), the control circuit <NUM> controls the switch S5 to open and the ADC2 circuit <NUM> of <FIG> samples and converts the voltage on the node Vamp, or G*(VDAC1_2-Ve+n), with the digital result being DADC2_3. The final result is the combination of the DAC word of DAC1 circuit <NUM> and outputs of the ADC circuit <NUM>. As before, VIN is given by the following equations: <MAT>.

In some example configurations, the DAC1 value can be modified, e.g., by adjusting the lower order bits, to reduce the voltage at the node Vamp to a value close to zero for the final ADC2 conversion. This optional operation can reduce the effect of any error in the amplifier gain G.

In another example configuration, the DAC1 value can be modified instead, e.g., by adjusting the lower order bits, to make the voltage Vamp_3 close to the voltage Vamp_1. This optional operation can reduce the resolution required for ADC2 circuit <NUM> by modifying ADC2 to convert the smaller amount
Vamp_3 -Vamp_1.

The converter circuit <NUM> can optionally include an auxiliary ADC circuit ADC3, which can include two sampling capacitors C3 and C4 and two corresponding sampling switches S6 and S7, and an adder circuit <NUM>. Table <NUM> depicts the operation of this example:.

The operations at T1 and T2 are identical to Table <NUM>, except that an additional switch S6 is closed.

At time T3, the control circuit <NUM> controls the switch S1 to close. Both ADC2 and ADC3 (using C3) acquire the voltage at the node of Vamp.

At time T4, the control circuit <NUM> controls the switches S5 and S6 to open simultaneously to sample the Vamp voltage onto the capacitors C2 and C3. The sampled voltage of Vamp is Vamp_1.

At time T5, the control circuit <NUM> controls the switches S3 and S7 to close and the switch S1 to open. In this phase, the voltage at node Vamp is Vamp_2, which equals G*(VDAC1_1-Ve+n). The DAC logic circuit <NUM> can load the DAC circuit <NUM> using the output DADC1 of the ADC1 circuit <NUM> and the DAC1 circuit <NUM> can generate an output voltage VDAC1_1, which is the voltage at the node Vcap.

At time T6 (row <NUM>), the control circuit <NUM> controls the switch S7 to open and the ADC circuit ADC3 samples the voltage on the node Vamp, where Vamp_2=G*(VDAC1_1-Ve+n). Then the voltage difference stored on C3 and C4 in ADC3, Vamp_2-Vamp_1, is converted to the digital result being DADC3.

At time T7, a new DAC1 code, DDAC1_2, is generated based on DADC3 and DDAC1_1. The control circuit <NUM> can apply the new code DDAC1_2 to DAC1, and the Vcap voltage equals VDAC1_2. The voltage of Vamp is Vamp_3 and can be adjusted closer to Vamp_1 by the code DDAC1_2. The capacitor C2 left side voltage is shifted from Vamp_1 to Vamp_3, and its right side voltage is shifted from <NUM> to Vamp_3-Vamp_1. Because both Vamp_3 and Vamp_1 contain the noise "n", the subtraction result does not contain the noise "n". At time T8, the ADC circuit ADC2 right side voltage, Vamp_3-Vamp_1, is converted by means of a SAR algorithm, or another gain stage and converter, etc., to a digital code DADC2, and DDAC1_2 is combined with DADC2 to generate the final ADC result. In optional phases of the conversion process, other noise can be filtered, e.g., using a dynamic filter, to reduce a noise bandwidth of the difference signal (generated between the estimated value of the analog input signal and for example, the equivalent voltage on the node VCAP during the conversion process) amplified by amplifier circuit <NUM>. As shown in <FIG>, the ADC2 circuit <NUM> can include a dynamic filter that can include a resistive component R1, and a bypass switch <NUM> to bypass the resistive component R1, as described above with respect to <FIG> and <FIG>.

Ideally, to provide the smallest difference signal, VIN_ESTIMATE of the analog input signal (or its equivalent) should be very nearly equal to VIN when the switch S5 opens (or ADC2 takes its first sample). A small difference signal allows a higher amplifier gain G and/or a wider bandwidth or dv/dt input signal. In some example configurations, the value of VIN_ESTIMATE can be the same as the value of VIN when the switch S4 opens. However, the switch S4 opens some time before the switch S5 (or ADC2 samples), during which time VIN may have moved to a new value. It can be desirable to account for the change in VIN over this period. In the example configuration shown in <FIG> and <FIG> below, a DAC2 circuit has been added to provide a better equivalent value for VIN_ESTIMATE.

The values that DAC2 can be set to can be based on: <NUM>) additional measurements of the input signal and/or its derivative (which may require additional ADCs); <NUM>) previous conversion results from either the final converter result or conversion results from sub-ADCs such as ADC1 or ADC2; or <NUM>) on knowledge of the signal that is being converted. For an oversampled or slow-moving signal, the value of the difference signal (VIN - VIN_ESTIMATE) may not change substantially from conversion to conversion. The measured value of the difference signal can then be used to provide an improved value for DAC2 for the next conversion. By using the history from several previous conversions, higher order derivatives can be calculated and used to determine even better predictions for VIN_ESTIMATE.

It should be noted that although the prediction circuitry is described in <FIG> and <FIG> with respect to a particular ADC, the prediction circuitry techniques of this disclosure are not limited to this ADC circuit. Rather, the prediction techniques are applicable to each of the ADC circuits described in this disclosure.

<FIG> is a simplified schematic diagram of the ADC circuit <NUM> of <FIG> including an example of a prediction circuit. The ADC circuit <NUM> of <FIG> can include components similar to those shown in <FIG>, with like elements indicated by like reference numerals. In addition, the ADC circuit <NUM> can include a VIN prediction calculation circuit <NUM> coupled to a DAC circuit <NUM> ("DAC2"). The prediction circuitry of <FIG> can predict a value of the analog input signal at or close to the sampling instance to provide an improved estimated value of the input signal VIN_ESTIMATE.

In the example shown in <FIG>, the DAC2 circuit can generate the entire VIN_ESTIMATE voltage. The VIN_ESTIMATE value for DAC2 may be derived by linear interpolation from the two previous conversion results. For example, the VIN prediction calculation circuit <NUM> can determine a slope (dVIN /dt) of a line using two previous conversion results.

As mentioned above, higher order derivatives can be determined and used to improve the prediction of the VIN_ESTIMATE voltage. For example, in some example configurations, the VIN prediction calculation circuit <NUM> can determine a rate of change of the slope (d<NUM>VIN /dt<NUM>) to generate the VIN_ESTIMATE voltage. In other example configurations, the VIN prediction calculation circuit <NUM> can determine a value of d<NUM>VIN /dt<NUM>, which is the rate at which the rate of change of the slope is changing.

<FIG> is a simplified schematic diagram of the ADC circuit <NUM> of <FIG> including another example of a prediction circuit. The ADC circuit <NUM> of <FIG> can include components similar to those shown in <FIG>, with like elements indicated by like reference numerals.

In addition, the prediction circuitry of the ADC circuit <NUM> can include a delta VIN prediction calculation circuit <NUM> coupled to a DAC circuit <NUM> ("DAC2"). The prediction circuitry of the ADC circuit <NUM> can further include a capacitor circuit C3 that can be coupled either to the DAC2 circuit via a switch S6 or to a bias voltage or ground via a switch S7. The prediction circuitry of <FIG> can predict a value of the analog input signal at or close to the sampling instance to provide an improved estimated value of the input signal VIN_ESTIMATE.

The DAC2 circuit can be used to account for changes in VIN between when VIN_ESTIMATE is sampled by the switch S4 (e.g., time T2 in Table <NUM>) and when the ADC2 circuit samples the voltage Vamp (e.g., time T4 in Table <NUM>). The voltage of VDAC2 can be calculated using Equation <NUM>: <MAT>.

The delta VIN prediction calculation circuit <NUM> can determine a slope (DVIN /dt) of a line using two previous conversion results.

In some example implementations, the DAC2 circuit can be updated several times between when S4 opens and when the ADC2 circuit takes its first conversion to minimize the magnitude of the signal being processed by the amplifier circuit <NUM> over this period. In this example implementation VIN_ESTIMATE can be thought of as a combination of VIN sampled at (T2) and the value VDAC2 being provided by DAC2 circuit.

In operation, after opening the switch S4 and prior to opening switch S5, the control circuit <NUM> opens the switch S6, capturing the charge VDAC2 (based on the delta VIN prediction calculator circuit <NUM>) on C3. On closing the switch S7, the voltage change of VDAC2 on the node Vcap3 is transferred to the input of amplifier <NUM>, depending on the capacitor ratio between C3 and C1.

To minimize or eliminate any gain error caused by the amplifier circuit <NUM>, it is desirable that the voltage on the node Vamp is close to zero when S5 opens. The prediction circuitry of <FIG> can change the voltage at the input node <NUM> of the amplifier circuit <NUM>, over a period of time, such that this voltage and hence the the voltage on VAMP are close to zero volts. Using these techniques, when ADC2 samples the difference signal, the value at the input node <NUM> of the amplifier circuit <NUM> is close to zero volts.

The drawings show, by way of illustration, specific embodiments in which the invention may be practiced. " Such examples may include elements in addition to those shown or described.

Method examples described herein may be machine or computer-implemented at least in part. Some examples may include a computer-readable medium or machine-readable medium encoded with instructions operable to configure an electronic device to perform methods as described in the above examples. An implementation of such methods may include code, such as microcode, assembly language code, a higher-level language code, or the like. Such code may include computer readable instructions for performing various methods. Further, in an example, the code may be tangibly stored on one or more volatile, non-transitory, or non-volatile tangible computer-readable media, such as during execution or at other times. Examples of these tangible computer-readable media may include, but are not limited to, hard disks, removable magnetic disks, removable optical disks (e.g., compact discs and digital video discs), magnetic cassettes, memory cards or sticks, random access memories (RAMs), read only memories (ROMs), and the like.

Claim 1:
A method of operating an analog-to-digital converter, ADC, circuit (<NUM>) to convert an analog input signal, the ADC circuit including a first capacitor circuit (C1) and a converter circuit (<NUM>), wherein the converter circuit comprises an amplifier (<NUM>), and wherein a first terminal of the first capacitor circuit (C1) is coupled to an ADC input (<NUM>) and a second terminal of the first capacitor circuit (C1) is coupled to an input of the amplifier (<NUM>), the method comprising:
opening a first switch (S4) coupled between the second terminal of the first capacitor circuit (C1) and a bias voltage to decouple the first capacitor circuit (C1) from the bias voltage while the first terminal of the first capacitor circuit (C1) is receiving an estimate value, wherein the estimate value is a value of the input signal that is estimated to occur at or close to a subsequent sampling instance, such that the estimate value is sampled across the first capacitor circuit (C1);
after opening the first switch (S4), opening a second switch (S5) in the converter circuit at the subsequent sampling instance while the first terminal of the first capacitor circuit (C1) is receiving the analog input signal, wherein the opening of the second switch samples a representation of an amplified combination of a noise voltage and a difference signal that is generated between the sampled estimate value and the analog input signal across a second capacitor circuit (C2), the second capacitor circuit (C2) coupled to an output of the amplifier (<NUM>);
generating a digital output using the sampled representation of the amplified combination of the noise voltage and the difference signal; and
converting the digital output to an analog signal representative of the digital output using a digital to analog converter circuit (<NUM>), and feeding back and coupling the analog signal representative of the digital output to the first terminal of the first capacitor circuit (C1) to adjust a voltage applied to the first terminal of the first capacitor circuit (C1) for performing a successive approximation search, wherein the ADC input is decoupled from the first terminal of the first capacitor circuit (C1) before coupling the analog signal representative of the digital output to the first terminal of the first capacitor circuit (C1).