SAR pipeline analog-to-digital converter (ADC) with foreground self-calibration
A pipelined SAR ADC that is connected to receive an analog input voltage includes a first ADC stage, a residue amplifier, a second ADC stage, and calibration circuitry coupled and configured to add first and second ADC conversion results to form an uncalibrated digital value, and to apply calibration values to the uncalibrated digital value to obtain a calibrated digital value corresponding to a calibrated digital representation of the analog input voltage, where the calibration values include a first ADC stage DAC element mismatch calibration value corresponding to the first ADC conversion result, an offset calibration value, and a gain adjustment factor.
1 . A pipelined successive approximation register (SAR) analog-to-digital converter (ADC) comprising:
a front-end ADC stage comprising a front-end digital-to-analog converter (DAC) connected to receive an analog input voltage and configured to output a residue voltage signal and to generate a first ADC conversion result by sampling and converting the analog input voltage with a front-end SAR circuit block;
a residue amplifier connected to receive the residue voltage signal and to generate an amplified residue voltage signal;
a back-end ADC stage comprising a back-end DAC connected to receive the amplified residue voltage signal and configured to generate a second ADC conversion result by sampling and converting the amplified residue voltage signal with a back-end SAR circuit block; and
calibration circuitry coupled and configured to add the first and second ADC conversion results to form an uncalibrated digital value, and to apply a plurality of calibration values to digitally adjust the uncalibrated digital value to obtain a calibrated digital value corresponding to a calibrated digital representation of the analog input voltage,
where the plurality of calibration values comprises:
a DAC element mismatch calibration value corresponding to the first ADC conversion result,
an offset calibration value comprising a residue amplifier offset value and a back-end ADC stage offset value, and
a gain adjustment factor.
2 . The pipelined SAR ADC of claim 1 , where the front-end ADC stage comprises:
a plurality of DAC elements and a termination element in the front-end DAC coupled to receive the analog input voltage, a high reference voltage, and a low reference voltage, and to provide a first output voltage;
a first comparator having a first input to receive the first output voltage of the front-end DAC, a second input, and a first comparator output, where the first comparator is configured to generate a first comparison output based on a difference in voltage between the first input and the second input; and
successive-approximation-register (SAR) circuitry having an input coupled to receive the first comparison output, and a first SAR output to provide the uncalibrated digital value corresponding to an uncalibrated digital representation of the analog input voltage.
3 . The pipelined SAR ADC of claim 1 , where the back-end ADC stage comprises:
a plurality of DAC elements and a termination element in the back-end DAC coupled to receive the amplified residue voltage, the high reference voltage, and the low reference voltage, and to provide an second output voltage;
a second comparator having a first input to receive the second output voltage of the back-end DAC, a second input, and a second comparator output, where the second comparator is configured to generate a second comparison output based on a difference in voltage between the first input and the second input; and
successive-approximation-register (SAR) circuitry having an input coupled to receive the second comparison output, and a second SAR output to provide the uncalibrated digital value corresponding to an uncalibrated digital representation of the amplified residue voltage.
4 . The pipelined SAR ADC of claim 1 , where the calibration circuitry is configured to apply the plurality of calibration values to the uncalibrated digital value by computing a difference value by subtracting the DAC element mismatch calibration value and the residue amplifier offset calibration value from the uncalibrated digital value, and then multiplying the gain adjustment factor with the difference value to obtain the calibrated digital value.
5 . The pipelined SAR ADC of claim 1 , wherein the calibration circuitry is configured to compute the offset calibration value by shorting together and then releasing the inputs to the residue amplifier, and then sampling and converting the amplified residue voltage with the back-end ADC stage to generate the second ADC conversion result which contains the offset calibration value.
6 . The pipelined SAR ADC of claim 1 , wherein the front-end DAC comprises a plurality of DAC elements, and wherein the calibration circuitry is configured to obtain a DAC element mismatch calibration value for each DAC element in the plurality of DAC elements of the front-end DAC by using the front-end SAR circuit block to individually compare a capacitance of each selected DAC element to a combination of zero or more other DAC elements in the front-end DAC in a way that limits the residue voltage and avoids saturating the back-end ADC.
7 . The pipelined SAR ADC of claim 6 , wherein the calibration circuitry is configured to generate the DAC element mismatch approximation value for each individual DAC element by using the back-end SAR ADC circuit block to perform successive approximation of the amplifed residue voltage signal.
8 . A foreground self-calibration method for calibrating a pipelined successive approximation analog-to-digital converter (SAR ADC) comprising a front-end ADC stage connected over a residue amplifier to a back-end ADC stage, the foreground self-calibration method comprising:
computing an offset calibration value for the residue amplifier and back-end ADC stage by using the back-end ADC stage to provide a digital measure of the offset voltage of the residue amplifier and back-end ADC stage;
computing an element mismatch calibration value for each digital-to-analog converter (DAC) element in a front-end DAC of the front-end ADC stage by switching a plurality of DAC elements in the front-end DAC to generate a residue voltage input to the residue amplifier that is roughly in the range of −1 to +1 least significant bit (LSB) of the front-end ADC stage and using the back-end ADC stage to measure an amplified residue voltage generated by the residue amplifier after for each selected DAC element being calibrated;
computing one or more summed calibration values for one or more first ADC conversion results based on one or more of the computed element mismatch calibration values; and
computing a gain adjustment value based on a total sum of all DAC elements in the front-end ADC stage that are used to sample an analog input voltage.
9 . The foreground self-calibration method of claim 8 , further comprising:
generating first and second ADC conversion results from the front-end ADC stage and back-end ADC stage to form an uncalibrated SAR ADC result;
subtracting, from the uncalibrated SAR ADC result, the offset calibration value and a summed calibration value selected from the one or more summed calibration values corresponding to the first ADC conversion result to form a difference value; and
multiplying the gain adjustment factor with the difference value to obtain the calibrated SAR ADC result from the pipelined SAR ADC.
10 . The foreground self-calibration method of claim 8 , where computing the offset calibration value comprises tri-stating input terminals into the residue amplifier to create a residue voltage that the residue amplifier amplifies for sampling and conversion by the back-end ADC stage to provide the digital measure of the offset voltage of the residue amplifier and back-end ADC stage.
11 . The foreground self-calibration method of claim 8 , where computing the element mismatch calibration value comprises:
measuring each single unit value DAC element being calibrated in the front-end DAC by sampling a high reference voltage Vrefh on the single unit value DAC element, and then switching the single unit value DAC element being calibrated to a low reference voltage Vrefl to create a residue voltage Vres that is amplified by the residue amplifier for sampling and conversion by the back-end ADC stage.
12 . The foreground self-calibration method of claim 11 , where measuring each single unit value DAC element being calibrated in the front-end DAC comprises a sampling phase comprising:
charging all top capacitor plates of the plurality of DAC elements to a common mode voltage;
charging a bottom capacitor plate of the DAC element being calibrated to a high reference voltage Vrefh;
charging all bottom capacitor plates of the plurality of DAC elements except the DAC element being calibrated to a low reference voltage Vrefl; and
disconnecting all top capacitor plates of the plurality of DAC elements from the common mode voltage at the end of the sample phase.
13 . The foreground self-calibration method of claim 12 , where measuring each single unit value DAC element being calibrated in the front-end DAC comprises an approximation phase comprising:
charging the bottom capacitor plate of the DAC element being calibrated to the low reference voltage Vrefl; and
using the back-end ADC stage to run a successive approximation measure on the amplified residue voltage generated by the residue amplifier to obtain a first approximation result for the element mismatch calibration value for the single unit DAC element being calibrated in the front-end DAC.
14 . The foreground self-calibration method of claim 8 , where computing the element mismatch calibration value comprises:
measuring each multi-unit value DAC element being calibrated in the front-end DAC by sampling a high reference voltage Vrefh on the multi-unit value DAC element, and then switching the multi-unit value DAC element being calibrated to a low reference voltage Vrefl to create a residue voltage Vres that is amplified by the residue amplifier for sampling and conversion by the back-end ADC stage.
15 . The foreground self-calibration method of claim 14 , where measuring each multi-unit value DAC element having a capacitance value of k*C being calibrated in the front-end DAC comprises a sampling phase comprising:
charging all top capacitor plates of the plurality of DAC elements to a common mode voltage;
charging a bottom capacitor plate of the multi-unit DAC element having a capacitance value of k*C to a high reference voltage Vrefh;
charging all bottom capacitor plates of the plurality of DAC elements having a capacitance value of (k−1)*C to a low reference voltage Vrefl; and
disconnecting all top capacitor plates of the plurality of DAC elements from the common mode voltage at the end of the sample phase.
16 . The foreground self-calibration method of claim 15 , where measuring each multi-unit value DAC element being calibrated in the front-end DAC comprises an approximation phase comprising:
charging the bottom capacitor plate of the multi-unit DAC element having a capacitance value of k*C to the low reference voltage Vrefl;
charging all bottom capacitor plates of the plurality of DAC elements having a capacitance value of (k−1)*C to the high reference voltage Vrefh; and
using the back-end ADC stage to run a successive approximation measure on the amplified residue voltage generated by the residue amplifier to obtain a first approximation result for the element mismatch calibration value for the multi-unit DAC element being calibrated in the front-end DAC.
17 . The foreground self-calibration method of claim 8 , wherein the plurality of DAC elements in the front-end DAC comprises a plurality of DAC capacitors, a termination DAC capacitor, and one or more redundant DAC capacitors.
18 . A pipelined successive approximation register analog-to-digital converter (SAR ADC) comprising a front-end ADC connected over a residue amplifier to a back-end ADC:
wherein the front-end ADC comprises:
a first digital-to-analog converter (DAC) configured to receive a first input voltage signal, a high voltage reference, and a low voltage reference, and to generate therefrom a first DAC output, wherein the first DAC comprises a plurality of bits encoded with one or more first DAC capacitors,
a first comparator having a first input coupled to receive the first DAC output, a second input, and a first comparator output, the first comparator configured to generate a differential comparison of the first and second inputs at the first comparator output, and
a successive-approximation-register (SAR) logic and calibration control block having an input coupled to receive the first comparator output, wherein the SAR logic and calibration control block is configured to provide a first ADC conversion result representation of the analog input voltage and to generate feedback control signals to the first DAC; and
wherein the back-end ADC comprises:
a second DAC configured to receive an amplified residue voltage from the residue amplifier, a high voltage reference, and a low voltage reference, and to generate therefrom a second DAC output, wherein the second DAC comprises a plurality of bits encoded with one or more second DAC capacitors, and
a second comparator having a first input coupled to receive the first DAC output, a second input, and a second comparator output, the second comparator configured to generate a differential comparison of the first and second inputs at the second comparator output;
wherein the SAR logic and calibration control block is configured to provide control signals to the front-end ADC and back-end ADC to measure an offset calibration value for the residue amplifier and back-end ADC by tri-stating input terminals into the residue amplifier to create a residue voltage that the residue amplifier amplifies for sampling and conversion by the back-end ADC.
19 . The pipelined SARADC of claim 18 , wherein the SAR logic and calibration control block is configured to provide control signals to the front-end ADC and back-end ADC to:
measure an element mismatch calibration value for each of the one or more first DAC capacitors in the first DAC by switching a plurality of DAC capacitors in the first DAC to generate a residue voltage input to the residue amplifier that is roughly in the range of −1 to +1 least significant bit (LSB) of the front-end ADC and using the back-end ADC to measure an amplified residue voltage generated by the residue amplifier after for each selected DAC capacitor being calibrated.
20 . The pipelined SARADC of claim 19 , wherein the SAR logic and calibration control block is configured to:
compute one or more summed calibration values for one or more first ADC conversion results based on one or more of the measured element mismatch calibration values;
compute a gain adjustment value based on a total sum of all DAC capacitors in the front-end ADC that are used to sample the analog input voltage.