Current takes the easiest path

All currents travel through a circuit and return to their source, but the path home isn't always the preferred one. When current travels along undesirable paths, it may cause unacceptable EMI emissions in circuits. In a demonstration at the IEEE EMC Symposium (August 19–21 2003, Boston, MA), Elya Joffe of KTM Project Engineering (Kfar-Sava, Israel) showed that at frequencies above 1 MHz, current will follow the path of least inductance to its source.

With a simple experiment, you can verify that high-frequency current takes the path of least inductance back to its source. Courtesy of Elya Joffe.

To prove his point, Joffe constructed a loop (see figure) that contains two possible current paths. One path runs through the cable loop. A copper strip "shorts" the loop, thus creating a second path. Joffe placed the loop in the return path from a load resistor to a function generator and then measured the current through the cable and through the strip with a current probe and a spectrum analyzer.

The spectrum analyzer plot revealed that at low frequencies, all of the current passes through the ground strap to return to its source. At high frequencies, all of the current passes through the cable.

To learn more about why current flows through the path of least inductance, you can download Joffe's presentation (PDF format). The presentation also discusses how return paths affect circuit performance and create EMI emissions. You'll learn how to mitigate these problems through grounding, shielding, bypassing, and layout techniques.