A 45Gb/s PAM-4 Transmitter Delivering 1.3Vppd Output Swing with 1V supply in 28nm CMOS FDSOI

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Abstract
Pushed by the ever-increasing demand of high-speed connectivity, next generation 400Gb/s electrical links are targeting PAM-4 modulation to limit channel loss and preserve link budget. Compared to NRZ, a higher amplitude is desirable to counteract the 1/3 reduction of PAM-4 vertical eye opening. However, linearity is also key, and PAM-4 levels must be precisely spaced to preserve the horizontal eye opening advantage it has over NRZ. This paper presents a 45Gb/s PAM-4 transmitter able to deliver a very large output swing with enhanced linearity and state-of-the-art efficiency. Built around a hybrid combination of current-mode and voltage-mode topologies, the driver is embedded into a 4-taps 5-bits FFE, and allows tuning the output impedance to ensure good source termination. Implemented in 28nm CMOS FDSOI process, the full transmitter includes a half-rate serializer, duty-cycle correction circuit, >>2kV HBM ESD diodes, and delivers a full swing of 1.3Vppd at 45Gb/s, while drawing 120mA only from 1V supply. The power efficiency is ~2 times better than previously reported PAM-4 transmitters.

Related Publications
[1] M. Bassi, F. Radice, M. Bruccoleri, S. Erba and A. Mazzanti, “A High-Swing 45 Gb/s Hybrid Voltage and Current-Mode PAM-4 Transmitter in 28 nm CMOS FDSOI,” in IEEE Journal of Solid-State Circuits, vol. 51, no. 11, pp. 2702-2715, Nov. 2016.
[2] M. Bassi, F. Radice, M. Bruccoleri, S. Erba and A. Mazzanti, “3.6 A 45Gb/s PAM-4 transmitter delivering 1.3Vppd output swing with 1V supply in 28nm CMOS FDSOI,” 2016 IEEE International Solid-State Circuits Conference (ISSCC), San Francisco, CA, 2016, pp. 66-67.