A High-Swing 45 Gb/s Hybrid Voltage and Current-Mode PAM-4 Transmitter in 28 nm CMOS FDSOI

— Pushed by the ever-increasing demand of high-speed
connectivity, next generation 400 Gb/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 45 Gb/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 feed-forward equalizer (FFE),
and allows tuning the output impedance to ensure good source
termination. Implemented in 28 nm CMOS FDSOI process,
the full transmitter includes a half-rate serializer, duty-cycle
correction circuit, >> 2 kV HBM ESD diodes, and delivers a
full swing of 1.3 Vppd at 45 Gb/s, while drawing only 120 mA
from 1 V supply. The power efficiency is ∼ 2 times better than
previously reported PAM-4 transmitters