cmrr

CMRR stands for common-mode rejection ratio. It is a key specification for differential amplifiers, instrumentation amplifiers, and related circuitry that amplifies the difference between two input signals while rejecting signals that appear on both inputs equally. A high CMRR indicates strong suppression of common-mode interference such as power-line hum, ground loops, and other ambient noise, improving measurement accuracy.

The CMRR is defined as the ratio of the differential-mode gain (Ad) to the common-mode gain (Ac): CMRR = Ad/Ac. In decibels, CMRR is expressed as CMRR(dB) = 20 log10(|Ad/Ac|). A device with identical differential and common-mode gains would have a CMRR of 1 (0 dB); as the common-mode gain approaches zero relative to the differential gain, the CMRR increases.

Measurement of CMRR typically involves applying a known common-mode signal to both inputs while keeping the differential input at zero, then measuring the output. The ratio of the output due to the differential signal to that due to the common-mode signal (or the ratio Ad/Ac) determines the CMRR. CMRR is frequency dependent and often degrades at higher frequencies due to parasitics and the finite bandwidth of internal stages.

Typical values vary by device and frequency. Modern precision instrumentation amplifiers and op-amps commonly report CMRR in the tens to hundreds of decibels across their specified bandwidths. Factors that influence CMRR include resistor matching in differential pairs, open-loop gain, temperature drift, layout parasitics, and the input common-mode range.

Applications include precise sensor interfacing (bridge sensors, strain gauges, thermocouples), biomedical instrumentation (EEG, ECG), and any system requiring robust rejection of common-mode noise to accurately recover small differential signals.