## EMC Question of the Week: September 27, 2021

The common-mode rejection ratio of an ideal differential receiver is

- 0
- 0 dB
- 6 dB
- infinite

## Answer

The correct answer is “d.” The gain a differential amplifier is the ratio of its output voltage to its differential input voltage, V_{+} - V_{-}, when there is no common-mode voltage on the input. This could also be referred to as the differential voltage gain. The common-mode gain of a differential amplifier is the ratio of its output voltage to the common-mode input voltage, (V_{+}+V_{-})/2, when there is no differential-mode voltage on the input. The common mode rejection ratio is the ratio of the differential-mode gain to the common-mode gain. In an ideal differential receiver, the common-mode gain is 0, so the common mode rejection ratio is infinite.

Note that if one side of the differential receiver's input is shorted to ground, the differential-mode gain and the common-mode gain are equal. In this case, the common mode rejection ratio is "1" or 0 dB.

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