A 48-Gb/s Inductorless PAM-4 Optical Receiver With 1.28-pJ/bit Efficiency in 28-nm CMOS

This work presents a 48-Gb/s four-level pulse amplitude modulation (PAM-4) optical receiver (ORX) with a linear analog front-end (AFE) and an integrated sampler. The AFE employs a transadmittance-stage transimpedance-stage (TAS-TIS) topology, replacing conventional CML-based variable gain amplifiers (VGAs) and post-amplifiers, avoiding continuous-time linear equalizers and passive inductors while preserving linearity and gain-bandwidth product for PAM-4 operation. The sampler incorporates a two-tap feedforward equalizer (FFE) and a two-tap decision feedback equalizer (DFE) to mitigate intersymbol interference from the AFE, ensuring correct data recovery. The timing criteria of the DFE loop are met at speeds up to 30 Gbaud by optimizing the clock-to-Q delay of the slicers. Fabricated in a 28-nm bulk CMOS process, the ORX is wire-bonded to a commercial photodiode (PD). Optical measurement results at 48-Gb/s PAM-4 demonstrate that the ORX achieves −5.1 dBm sensitivity at a 2.4e-4 bit error rate (BER), consuming 61.4 mW, with only 13.1 mW contributed from the AFE, resulting in 1.28-pJ/bit (0.27 pJ/bit for AFE alone) energy efficiency.