Resistor matching is important in FDAs to maintain good output balance. An ideal differential output signal implies the two outputs of the FDA should be exactly equal in amplitude and shifted 180° in phase. Any imbalance in amplitude or phase between the two output signals results in an undesirable common-mode signal at the output. The output balance error is a measure of how well the outputs are balanced and is defined as the ratio of the output common-mode voltage to the output differential signal.

At low frequencies, resistor mismatch is the primary contributor to output balance errors. Additionally CMRR, PSRR, and HD2 performance diminish if resistor mismatch occurs. Therefore, to optimize performance, use 1% tolerance resistors or better. Table 9-1 provides the recommended resistor values to use for a particular gain.