A 0.17 pJ/bit 28 Gb/s/pin Single-Ended PAM-4 Transmitter for On-Chip Short-Reach Unterminated Channels

This paper presents the design of a single-ended four-level pulse-amplitude modulation (PAM-4) transmitter for an on-chip short-reach unterminated channel. To achieve multi-output generation, a local voltage buffer consisting of a diode-connected device and a leaker transistor is introduced. By charge-sharing between a local reservoir capacitor and an unterminated channel, the proposed transmitter generates mid-level output voltages without using the DC current, thereby realizing multi-level signaling without significantly increasing the static current. A prototype chip was fabricated by 28 nm CMOS process, and the transmitter exhibits an energy efficiency of 0.17 pJ/bit at 28 Gb/s/pin, which is state-of-the-art energy efficiency as a multi-level transmitter having a data rate beyond 20 Gb/s.