Common-mode rejection ratio (CMRR)

The common-mode rejection ratio measures a differential front-end's ability to ignore signals that appear equally on both of its inputs while amplifying the difference between them. It is the ratio of differential gain to common-mode gain, expressed in decibels; neural front-ends typically target at least 60 to 80 dB or more. The reason it matters so much is that the interference contaminating a microvolt neural recording — mains hum at 50 or 60 Hz, motion, and stimulation artifact — is overwhelmingly common-mode, appearing nearly identically on nearby electrodes.

In practice CMRR is limited by mismatch: differences in gain between the two input paths, and, crucially, mismatch in electrode impedance, which converts common-mode voltage into a differential error. So a high nominal amplifier CMRR can be squandered by unequal electrode impedances. A clean reference and bias-electrode scheme, and active feedback such as a driven-right-leg or active-reference loop that drives the body toward the amplifier common, push effective CMRR up.

Its close relative is the power-supply rejection ratio, the analogous rejection of noise on the supply rails. Without high CMRR, ubiquitous common-mode interference not only buries the neural signal but can drive the amplifier or ADC into saturation, so it is a first-order requirement of any recording front-end rather than a refinement.