LAUSR

This paper presents a PAM-4 receiver (RX) for CMOS image sensor (CIS) link. A robust baud-rate PAM-4 phase detector (PD) is proposed, which alleviates the dead zone issue encountered in the conventional sign-sign Mueller-Muller (SS-MM) PD when combined with an adaptive decision feedback equalizer (DFE). An optimum and unique locking point is reached by using a sign-sign minimum mean squared error (SS-MMSE) PD with a biased state which exhibits a constant probability for all phase error ranges. The proposed PD with the biased state evades the dead zone and prevents wandering. Furthermore, the phase-detecting transitions of the proposed SS-MMSE PD are 1.5 times higher compared to the conventional SS-MM PD. The proposed solution is verified with a prototype PAM-4 RX chip using 40-nm CMOS technology. It demonstrates a bit error rate (BER) of less than $10^{-9}$ with a 15.8 dB loss channel. The total power consumption is 46.6 mW at 12 Gb/s, achieving a figure of merit (energy efficiency per channel loss at Nyquist frequency) of 0.24 pJ/b/dB.