Bruce Archambeault, Distinguished Engineer, IBM
Samuel Connor, Senior Engineer, IBM
Joseph C. (Jay) Diepenbrock, Senior Technical Staff member, IBM

The use of so-called differential signaling for high-speed signals has become very common in today's high-speed system designs. While this signal strategy allows better data quality and signal integrity at the receive end of the traces and/or cables, there are some significant EMC and signal integrity issues that are not easily apparent. This paper demonstrates that a significant amount of common-mode current that can be created when the rise/fall times of the differential signals have small amounts of skew or are slightly different. These common-mode currents can have a significant impact on the EMC performance, causing significant levels of emissions unless careful design consideration is given to common-mode current return (ground return).