100–112-Gb/s 1.6-Vppd PAM-8 Transmitters With High-Swing 3 + 1 Hybrid FFE Taps in 40-nm Technology

This article presents two eight-level pulse amplitude modulation (PAM-8) transmitters (TX) that achieve 100- and 112-Gb/s data rates, and a high output swing of 1.6 peak-to-peak differential voltage (Vppd) in 40-nm CMOS technology. The high-voltage driver is adopted to enhance the output swing level with the protective cascode and current-bleeding techniques. The hybrid $3 {+} 1$ tap feed-forward equalizer (FFE) is implemented for efficient channel equalization. Two types of high-speed multiplexers are introduced for the final 4-to-1 serialization: a single-stack and single-stage multiplexer that achieves 33.3 Gbaud/s and a two-stage multiplexer at 37.3 Gbaud/s. Two prototype test chips are fabricated in 40-nm CMOS technology to evaluate the proposed multiplexer designs. The transmitters achieve the PAM-8 data rates of 100 Gb/s (for the single-stage multiplexer) and 112 Gb/s (for the two-stage multiplexer), with worst case eye-opening values of 45 and 57 mV. Their energy efficiencies are measured to be 3.35 and 4.56 pJ/bit.