Decoupling Power/Ground Planes

Abstract

It is likely that decoupling power and ground planes is one of the most misunderstood design concepts, and certainly the area having the most myths about the ‘correct’ decoupling strategy. The use of decoupling capacitors connected between the power and ground planes on a printed circuit board (PCB) is a common practice to help ensure proper functionality(i.e. signal integrity) and to reduce EMI emissions from printed circuit boards. The proper number and value of decoupling capacitors is always a topic of debate between EMC engineers and design engineers. Some typical rules-of-thumb include requiring a decoupling capacitor for each power pin on an IC, at least one decoupling capacitor per side of physically large ICs, and/or decoupling capacitors spread evenly over every square inch of the board. Few qualitatively proven approaches for the optimal approach to decoupling is available in the technical literature. These rules-of thumb can often result in drastic over-design of the decoupling strategy, since the saying ‘better safe than sorry’ is often applied. Many of these rules are really based in myth. In addition, there are a number of outright myths that exist and are published, causing significant confusion within the general design community.