Common-mode rejection ratio

In electronics, the common mode rejection ratio (CMRR) of a differential amplifier (or other device) is a metric used to quantify the ability of the device to reject common-mode signals, i.e. those that appear simultaneously and in-phase on both inputs. An ideal differential amplifier would have infinite CMRR, however this is not achievable in practice. A high CMRR is required when a differential signal must be amplified in the presence of a possibly large common-mode input, such as strong electromagnetic interference (EMI). An example is audio transmission over balanced line in sound reinforcement or recording. In electronics, the common mode rejection ratio (CMRR) of a differential amplifier (or other device) is a metric used to quantify the ability of the device to reject common-mode signals, i.e. those that appear simultaneously and in-phase on both inputs. An ideal differential amplifier would have infinite CMRR, however this is not achievable in practice. A high CMRR is required when a differential signal must be amplified in the presence of a possibly large common-mode input, such as strong electromagnetic interference (EMI). An example is audio transmission over balanced line in sound reinforcement or recording. Ideally, a differential amplifier takes the voltages, V + {displaystyle V_{+}} and V − {displaystyle V_{-}} on its two inputs and produces an output voltage V o = A d ( V + − V − ) {displaystyle V_{mathrm {o} }=A_{mathrm {d} }(V_{+}-V_{-})} , where A d {displaystyle A_{mathrm {d} }} is the differential gain. However, the output of a real differential amplifier is better described as where A c m {displaystyle A_{mathrm {cm} }} is the common-'mode gain', which is typically much smaller than the differential gain. The CMRR is defined as the ratio of the powers of the differential gain over the common-mode gain, measured in positive decibels (thus using the 20 log rule):