Common Mode Rejection Ratio, or CMRR, is a performance specification of an electronic circuit component called an operational amplifier, or op-amp. Essentially, it is a measure of how well an op-amp is able to ignore voltages that are common to both of its direct-current inputs, and thus describes how cleanly the op-amp amplifies signals that appear as voltage differences at its inputs.
What Op-Amps Do
An op-amp is designed to amplify voltage differences between its two power inputs. Because the op-amp is a direct-current (DC) component, its two inputs are positive voltage and negative voltage. The op-amp uses feedback circuitry to double the voltage difference between the two inputs, and send it to its single output. The amount of amplification is called the gain of the op-amps signal, and the amplification of the difference in the input voltages is called the differential gain.
In order to cleanly amplify voltage differentials, the op-amp has to be able to ignore voltage changes that are common to both of its power inputs. Any amplification of common voltage changes is called common-mode gain. A good op-amp, then, must be able to maximize differential gain while keeping common-mode gain to a minimum. It's ability to do this is called its Common Mode Rejection Ratio.
Calculating Common Mode Rejection Ratio
In order to calculate an op-amp's CMRR, you need to know the differential gain and the common mode gain of the op-amp. The differential gain is the voltage at the op-amps output, divided by the difference in voltage measured at the two inputs. The common mode gain is measured by referencing both inputs to a single voltage, and dividing the measured output voltage by that input voltage. The CMRR, then, is the differential gain divided by the common mode gain.
Importance of low CMRR
Because of imperfections in any real-world circuit, no op-amp has a functional CMRR of 0. However, low CMRRs are important in circuits in which the signal that is being measured is expressed as a voltage differential. In such circuits, voltages that are common to both inputs are seen as noise; the more they are amplified, the more the circuit will be prone to malfunction or inaccuracy. Low-CMRR op-amps were originally used in analog computers, and are used today in audio and video circuits to reduce noise.
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