3.4 A 36Gb/s PAM4 transmitter using an 8b 18GS/S DAC in 28nm CMOS

At data rates beyond 10Gb/s, most wireline links employ NRZ signaling. Serial NRZ links as high as 56Gb/s and 60Gb/s have been reported [1]. Nevertheless, as the rate increases, the constraints imposed by the channel, package, and die become more severe and do not benefit from process scaling in the same fashion that circuit design does. Reflections from impedance discontinuities in the PCB and package caused by vias and connectors introduce significant signal loss and distortions at higher frequencies. Even with an ideal channel, at every package-die interface, there is an intrinsic parasitic capacitance due to the pads and the ESD circuit amounting to at least 150fF, and a 50Ω resistor termination at both the transmit and receive ends resulting in an intrinsic pole at 23GHz or lower. In light of all these limitations, serial NRZ signaling beyond 60Gb/s appears suboptimal in terms of both power and performance. Utilizing various modulation techniques such as PAM4, one can achieve a higher spectral efficiency [2]. To enable such transmission formats, high-speed moderate-resolution data converters are required. This paper describes a 36Gb/s transmitter based on an 18GS/s 8b DAC implemented in 28nm CMOS, compliant to the new IEEE802.3bj standard for 100G Ethernet over backplane and copper cables [3].