6.6 A 128Gb/s 1.3pJ/b PAM-4 Transmitter with Reconfigurable 3-Tap FFE in 14nm CMOS

The ever-increasing demand for higher bandwidth continues to fuel the need for faster and more power-efficient IOs, with the next generation high-speed serial links expected to reach data rates higher than 112Gb/s using PAM-4 signaling [1–3]. While PAM-4 spectral efficiency is better than that of NRZ, it is less tolerant of residual ISI and noise. As a consequence, a driver with high bandwidth and large output amplitude is required. This paper presents a 64Gbaud PAM-4 TX with a fully reconfigurable 3-tap FFE, which achieves a power efficiency of 1.3pJ/b in PAM-4 mode and 2.7pJ/b in NRZ mode for a differential output swing of $1\mathrm{V}_{ppd}$. A feature of the FFE construction is the use of fully re-assignable FFE segments among the 3 taps, which allows a reduced number of segments for lower capacitance and higher driver bandwidth. To minimize power consumption, a quarter-rate clocking architecture is adopted with a tailless 4:1 multiplexer, which also acts as a pre-driver to a tailless CML output driver.