September 28, 2006 -- Circulating currents can wreak havoc for a design engineer, no matter whether your application is computers or communications. Some engineers lack an appreciation for circulating currents because of the schematic convention of using a ground or common symbol to show the return path for all the circuitry. Novice engineers often misinterpret this symbol as representing an ocean of zero impedance. Nothing could be further from the truth.

That ground symbol represents just another wire in your schematic. If the current in the ground connection is large enough or if it changes fast enough, it generates a significant amount of voltage. That voltage might interfere with the accuracy of a power supply. That voltage can also cause measurement errors in an instrumentation application. Digital-system engineers must grapple with ground bounce. Audio buffs see the effects of circulating currents in the dreaded ground loop that causes buzz and hum. RF engineers always struggle with controlling the flow of ground currents in high-frequency-system applications. Read on to find out the cause of circulating currents, get some real-world examples, and then learn solid design principles to keep these circulating currents from ruining your design.

By Paul Rako, Technical Editor, EDN