A 35-Gb/s PAM-4 Transmitter With 7B4Q Full-Transition Avoidance and Area-Efficient Gm-Boosting Techniques

This paper describes a channel-loss-tolerant 35-Gb/s PAM-4 transmitter with feed-forward equalization (FFE) for high-speed wireline interfaces. The proposed transmitter adopts 7B4Q full transition avoidance (FTA) coding in combination with the 2-tap FFE to improve the worst-case horizontal eye-opening in the presence of inter-symbol interference (ISI). The input and output bit widths (7-bit input and 8-bit (4Q) output) and the encoder structure are selected to achieve a high data-rate with a synthesizable encoder hardware. Additionally, an area-efficient gm-boosting voltage-mode driver is used to enhance the transition slope. The transmitter test chip was fabricated in a 28-nm CMOS process and occupied 0.18 mm. The design achieved 35-Gb/s with a 0.95x wider horizontal eye-opening by adopting the FTA coding for a 5.2-dB loss channel.