A 32 Gb/s ADC-based PAM-4 receiver with 2-bit/stage SAR ADC and partially-unrolled DFE

A PAM-4 ADC-based receiver employs a 32-way time-interleaved 6-bit 2-bit/stage loop-unrolled SAR ADC with a single capacitive reference DAC. Digital equalization complexity is reduced with a new PAM-4 DFE architecture that has a gate count comparable to an NRZ DFE, while simultaneously halving the critical path delay. A 3-tap FFE is embedded in the ADC using an additional non-binary DAC to improve the coverage of the 6-bit FFE coefficient space. This 3-tap embedded FFE and CTLE front-end partial equalization allows placement of the CDR's Mueller-Muller phase detector directly at the ADC output to avoid excessive loop delay. Fabricated in GP 65nm CMOS, the 32Gb/s receiver operates at a BER <; 10 ^-11 with a 27 dB loss channel and <; 10 ^-9 with a 30 dB loss channel without utilizing any transmit equalization. The complete ADC-based receiver achieves a power efficiency of 8.25pJ/bit, including all the front-end, ADC, and DSP power.