About Common-Mode Rejection

May 6, 2014

Common-mode rejection is the ability of the differential amplifier (which sits between the oscilloscope and probes as a signal-conditioning preamp) to eliminate the common-mode voltage from the output. Now, the ideal differential amplifier would remove all of the common-mode signal, or the voltage common to both sides of the differential pair.

Common-mode voltages can come from numerous sources, including an ambient radiated signal coupled into both lines, an offset from signal common created by the driver circuit, or a ground differential between the two ends of the differential circuit. Regardless of its cause, it’s not the common-mode voltage that’s of interest, but rather the differential voltage. Thus, the measure of how good the differential amplifier is at getting rid of common-mode voltage is its common-mode rejection ratio, or CMRR.

Be mindful that the CMRR of an amplifier changes with frequency. High-performing differential amplifiers start at very high CMRR ratings of as much as 100,000:1 and can maintain high CMRR values over a wide bandwidth. Meanwhile, high-voltage differential probes will provide good CMRR at dc. But as signal frequencies rise, CMRR deteriorates.

About the Author

David Maliniak

MWRF Executive Editor

In his long career in the B2B electronics-industry media, David Maliniak has held editorial roles as both generalist and specialist. As Components Editor and, later, as Editor in Chief of EE Product News, David gained breadth of experience in covering the industry at large. In serving as EDA/Test and Measurement Technology Editor at Electronic Design, he developed deep insight into those complex areas of technology. Most recently, David worked in technical marketing communications at Teledyne LeCroy. David earned a B.A. in journalism at New York University.