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# What is the purpose of using a differential amplifier? (Common-mode rejection ratio: CMRR)

Differential amplifiers are used mainly to suppress noise.

Noise consists of typical differential noise and common-mode noise, of which the latter can easily be suppressed with an op-amp.

There are two main causes of common-mode noise:

1. Noise is generated in the wires and cables, due to electromagnetic induction, etc., and it causes a difference in potential (i.e., noise) between the signal source ground and the circuit ground.

2. Current flowing into the ground of a circuit from another circuit causes a ground potential rise (noise).

In either case, the ground potential, a reference for a circuit, fluctuates because of noise. It is difficult to remove common-mode noise with typical filters. Differential amplifiers are used as a means of suppressing common-mode noise.

The op-amp configures this differential amplifier as the main circuit. The symbol shown below represents a differential amplifier. It has two inputs: VIN(+) and VIN(-). The output voltage is equal to a difference in voltage between the two inputs multiplied by the amp’s gain (AV):

VOUT=AV{VIN(+) - VIN(-)}

Suppose that common-mode noise (vnoise) is superimposed on the differential inputs. Then,

VIN(+)‘=VIN(+) + Vnoise

VIN(-)‘=VIN(-) + Vnoise

Hence, the output is expressed as follows. This indicates that the differential amplifier

cancels out common-mode noise:

VOUT=AV[{VIN(+) + Vnoise} - {VIN(-) + Vnoise}]

=AV{VIN(+) - VIN(-)}

The common-mode rejection ratio (CMRR) is specified as one of the electrical characteristics of an op-amp.

Figure Noise superimposed on the amplifier’s ground

Example of an electrical characteristics table in a datasheet

Also see the application notes on op-amps:
·Before creating and producing designs and using, customers must also refer to and comply with the latest versions of all relevant TOSHIBA information and the instructions for the application that Product will be used with or for.