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The high-speed, differential signals in todaypsilas multi-board systems often carry significant amounts of common mode current. Neglecting the common mode currents can produce unrealistic signal integrity and electromagnetic simulation results. A couple cases will be shown that illustrate the impact of common mode currents and mode conversion on the signal integrity of a link path. 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.