The high-speed differential signals in today's multi-board systems also carry significant amounts of common mode current. When the common mode currents flow through the inter-board connectors, the inductance of the common mode current return path in the connectors creates a noise voltage between the connected boards. The effect of this common mode noise on received differential signals is described in this paper using measurement and modeling results. Effective and accurate methods for quantifying the amount of common mode current, the amount of inductance in the common mode current return path, and the impact of the common mode noise on the differential signal are proposed to ensure better design practices.