6.3 A 40-to-56Gb/s PAM-4 receiver with 10-tap direct decision-feedback equalization in 16nm FinFET

The increasing bandwidth demand in data centers and telecommunication infrastructures had prompted new electrical interface standards capable of operating up to 56Gb/s per-lane. The CEI-56G-VSR-PAM4 standard [1] defines PAM-4 signaling at 56Gb/s targeting chip-to-module interconnect. Figure 6.3.1 shows the measured S21 of a channel resembling such interconnects and the corresponding single-pulse response after TX-FIR and RX CTLE. Although the S21 is merely ∼10dB at 14GHz, the single-pulse response exhibits significant reflections from impedance discontinuities, mainly between package and PCB traces. These reflections are detrimental to PAM-4 signaling and cannot be equalized effectively by RX CTLE and/or a few taps of TX feed-forward equalization. This paper presents the design of a PAM-4 receiver using 10-tap direct decision-feedback equalization (DFE) targeting such VSR channels.