The common-mode rejection ratio (CMRR) is one of the key elements when designing with fully differential amplifiers. Although FDAs are designed to provide the best CMRR performance, poor selection of external gain setting resistors, as well as careless board layout techniques, significantly degrade CMRR performance.

In an ideal world, the resistors in a typical circuit, as shown in the test circuit Figure 7-1, are chosen to be R_F1 / R_F2 = R_I1/R_I2. Mismatch between these ratios causes the differential output to depend on the input common-mode voltage (V_VOCM), and that in turn produces an offset and excess noise on the differential output. As mentioned in the previous section, the mismatch of the external resistor network primarily contributes to the dc error. Generally, a resistor mismatch of 0.1% and a ratio of 1 V/V results in a CMRR of 60 dB. The natural degradation of the external resistor network is minimized by the following guidelines:

• Consider input impedance matching, as shown in the Input impedance matching with fully differential amplifiers technical brief.
• Follow layout guidelines, as provided in Section 11.1.
• Use compensation techniques, as described in the Improving PSRR and CMRR in Fully Differential Amplifiers application report

Despite the mismatch of the external feedback network, the internal common-mode feedback amplifier regulates the outputs to remain balanced in amplitude and remain 180° out of phase. The output balance performance stays unaffected by the CMRR degradation.