Rate of rejection of the common mode

The rate of rejection of the common mode (TRMC, or CMRR for Common Mode Rejection Ratio ) is a numerical value quantifying the capacity of a Amplificateur differential to reject the common tension of its two entries. It is generally expressed in Décibel.

Definition

Ideally, an operational Amplifier is a Amplificateur differential, i.e. it amplifies the potential difference between its two entries. One has as a relation:

$V\_\; \{out\}\; =G\_d\; \backslash \; times\; (e\_+\; -\; e\_-)$

with $G\_d$ differential profit of the amplifier, V_out the output voltage of this amplifier, e₊ the tension applied to the non-inverseuse entry and e_- that applied to the inverseuse entry.

In practice, the output voltage of a AOP does not depend solely on the difference in tension between these two entries, it also depends on the median value of the tensions on these two entries (or tension of common mode). The entered relation left a AOP is established as follows:

$V\_\; \{out\}\; =G\_d\; \backslash \; times\; (e\_+\; -\; e\_-)\; +G\_\; \{mc\}\; \backslash \; frac\; \{e\_+\; +\; e\_-\}\; \{2\}$

with G_mc the amplification of common mode. In order to define the capacity of the amplifier to reject the common mode, one defines the “rate of rejection {, of, in, ø} common mode” (TRMC):

$TRMC=\backslash frac\{G\_d\}\{G\_\{mc\}\}$.

For a AOP, the TRMC uninterrupted varies between 70 and 130 dB following the type of amplifier but it strongly decreases with the increase in the frequency.

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