A high-gain differential-to-differential I/O circuit can be used to drive a second-stage FDA or a differential-to-single-ended stage. This circuit is frequently used in applications where high input impedance is required (for example, if the source cannot be loaded). Figure 8-3 illustrates an example design where the differential gain is 41 V/V. An added element between the two R_G resistors increases the noise gain for the common-mode feedback. Make sure to provision for the added element; otherwise, a decompensated VFA (such as the OPA838) often oscillates. With only the R_G elements in the differential I/O design, the common-mode feedback is unity-gain and often causes high-frequency, common-mode oscillations. To resolve this issue, split the R_G elements in half and add a low-impedance path, such as a capacitor or a DC reference, between the two R_G values.

Figure 8-4 High-Gain Differential I/O Stage

Integrated results are available, but the OPA838 provides a low-power, high-frequency result. For best CMRR performance, match the resistors. A good rule is CMRR is approximately equal to the resistor tolerance; therefore, a 0.1% tolerance provides approximately 60-dB CMRR.