Sensing and Cancellation Circuits for Mitigating EMI-Related Common Mode Noise in High-Speed PAM-4 Transmitter

The common mode (CM) current in differential circuits generates CM noise which radiates in the environment and causes electromagnetic interference (EMI) with electronic devices in proximity. This noise is generated due to imbalance in the charging and discharging paths of the driver circuit and asymmetric equalization. A systematic study of how asymmetric equalizer increase the CMnoise is still lacking. Few active on-chip solutions are proposed for wireline transmitters up to 20-Gbps speed. However, these solution does not suppress CM noise by the equalizer and they could not support four-level pulse amplitude modulation (PAM-4) optical transmitters. This paper analyzes two potential sources of EMI-related CM noise, the common mode logic (CML) driver and feed forward equalization (FFE) circuit, and presents a novel on-chip active circuit technique for automatic CM noise cancellation in high-speed PAM-4 transmitter. This solution provides the benefits of small size and low cost by eliminating the need for discrete components to suppress EMI. The post-layout simulation demonstrates that the CM noise cancellation circuit (CMNC) efficiently mitigate the EMI by suppressing the CM noise up to 90% and consumes 5 mW, with core area of 17 mu m x 9 mu m.