Pipelined SAR ADC using comparator as a voltage-to-time converter with multi-bit second stage

1. A two-stage successive-approximation-register (SAR) analog-to-digital converter (ADC) comprising:

a first stage comprising a SAR ADC, wherein the SAR ADC comprises a comparator;

a voltage-to-time interface that translates a voltage-domain residue from the SAR ADC to a time-domain residue, wherein the voltage-to-time interface comprises the comparator, the SAR ADC sharing the comparator with the voltage-to-time interface; and

a second stage comprising a time-to-digital converter (TDC) that resolves multiple bits from the time-domain residue.

2. The two-stage SAR ADC of claim 1 , wherein the second stage digitizes only a last residue of a particular input sample received at the first stage.

3. The two-stage SAR ADC of claim 1 , wherein the time-domain residue represents a resolving time of the comparator.

4. The two-stage SAR ADC of claim 1 , wherein the voltage-to-time interface further comprises an edge extractor (EE) circuit coupled to an output of the comparator, receiving a ready signal that is output from the comparator and a comparator clock signal.

5. The two-stage SAR ADC of claim 4 , wherein the second stage receives only a last residue of a particular input sample received at the first stage from an output of the EE circuit.

6. A method of operating a two-stage successive-approximation-register (SAR) analog-to-digital converter (ADC) comprising a first stage comprising a SAR ADC; a voltage-to-time interface that translates a voltage-domain residue from the SAR ADC to a time-domain residue; and a second stage comprising a time-to-digital converter (TDC) that resolves multiple bits from the time-domain residue, the method comprising:

sampling a first input voltage signal representing a plurality of bits at the first stage using a sampling clock;

converting to a first set of bits from the plurality of bits using the SAR ADC;

for a last bit cycle of the SAR ADC for the plurality of bits, translating the voltage-domain residue to the time-domain residue using the voltage-to-time interface; and

digitizing multiple bits from the time-domain residue using the TDC to output a second set of bits from the plurality of bits, and wherein digitizing multiple bits from the time-domain residue comprises:

determining a difference between a comparator clock signal and a ready signal output from a comparator of the voltage-to-time interface; and

quantizing the difference according to a resolution of the TDC; and

wherein the first set of bits with the second set of bits are combined to form the plurality of bits.

7. The method of claim 6 , wherein the voltage-to-time interface comprises a comparator, wherein the time-domain residue represents a resolving time of the comparator.

8. The method of claim 7 , wherein the voltage-to-time interface further comprises an edge extractor (EE) circuit coupled to an output of the comparator, the method comprising:

providing the time-domain residue from the voltage-to-time interface to the TDC for only the last bit cycle of the SAR ADC by using the EE circuit.

9. The method of claim 6 , wherein for the last bit cycle of the SAR ADC for the plurality of bits, outputting a comparison outcome of the SAR ADC as a polarity value for the second set of bits.

10. The method of claim 6 , wherein the plurality of bits comprises 6 to 14 bits.

11. The method of claim 6 , wherein the second set of bits comprises 4 to 8 bits.