An evaluation of single-ended and differential impedance in PCBs

Abstract

The electrical performance of pulses propagating in Printed Circuit Boards (PCBs) is determined by the characteristic impedance, differential impedance, and terminating impedance of both single-ended and differential pairs. In this paper, numerical values are evaluated for the characteristic impedance, differential impedance, and common mode impedance of microstrip and strip transmission lines embedded in a PCB stack up. These numerical values are also plotted for different values of frequency, ratios of signal width to signal height above a ground plane, and signal separation to signal height. Even and differential mode impedances are also evaluated and plotted for two common termination circuits used in routing peripheral connect interface bus (PCI), point-to-point low voltage differential signaling (LVDS), or multipoint to multipoint differential signaling bus (LVDSM). It is also shown that appropriate values of RC termination filters can be used effectively to match common mode and differential mode impedances thereby reducing EMC emissions, minimizing reflections, and providing stable differential input voltage swing levels. The plots aid in the design of differential signalling and controlled layer impedance.