AC-coupling strategy for high-speed transceivers of 10Gbps and beyond

AC coupling in a transmission link is preferred and often required for the functioning of high speed transceivers. But at data rate of 1OGbps and beyond, both the external AC coupling and the conventional on-chip AC coupling approaches bring in heavy burden that pushes to the fundamental limits and are difficult to afford. This paper examines the AC-coupling methods for multi-Gb/s transceivers, and points out the impairments in the existing implementations. A hybrid structure offering both the signal-bump and the AC-capacitor functions under the stringent return-loss requirements of a 1OGb/s+ I/O is proposed and implemented in 65nm standard CMOS. A sizeable 5.1pF AC capacitor is measured with ultra low parasitic expense ratio of less than 120fF.