A 28/56 Gb/s NRZ/PAM-4 Dual-Mode Transmitter with Eye-Opening Enhancement in 28 nm CMOS

This paper presents a non-return-to-zero (NRZ)/4-level pulse amplitude modulation (PAM-4) dual-mode wireline transmitter with an eye-opening enhancement technique to improve horizontal eye-opening. With the eye-enhancement pulse generator and the auxiliary pull-up device in the tail-less current-mode driver, the worst-case horizontal eye-opening increased by 30% in the PAM-4 eye diagram. The power efficiency of the NRZ mode was also improved by completely turning off the LSB path in the differential data path, resulting in only a 31% power efficiency degradation, which is far lower than that of the prior dual-mode transmitters. Fabricated in 28 nm CMOS, the transmitter achieves power efficiency of 1.4 pJ/bit at 56 Gb/s in PAM-4 mode and 1.84 pJ/bit at 28 Gb/s in NRZ mode, respectively.