Ground return for high speed digital signals that are capacitively

Abstract

A ground return scheme is provided for multiple high speed digital signals that are capacitively coupled between networks, where the networks are connected to different ground planes (i.e. they are isolated) on a common circuit board. A first embodiment of the invention provides a low impedance ground path for each signal crossing the isolation boundary. The ground path is achieved by placing a ground return capacitor proximate to the signal coupled capacitor to substantially minimize the loop area of the total current path. This arrangement substantially minimizes the self inductance of each signal current path and the mutual inductance, i.e. coupling, between signal paths. The arrangement also has the benefit of reducing self-induced voltage drops in each ground return path, thereby reducing crosstalk between the signals, reducing ground bounce (resulting in faster settling times), and also reducing common-mode voltage induced between the grounding systems. Consequently, application of the invention produces improved signal integrity, reduced generation of electromagnetic emissions, and reduced susceptibility to electromagnetic interference. A second embodiment of the invention adds common-mode chokes to each signal-ground pair. The chokes provide a measure of high-frequency isolation between the ground systems, as well as between the various signal paths.