Because the OPA690 is a unity-gain stable, voltage-feedback op amp, a wide range of resistor values can be used for the feedback and gain-setting resistors. The primary limits on these values are set by dynamic range (noise and distortion) and parasitic capacitance considerations. For a noninverting unity-gain follower application, make the feedback connection with a 25-Ω resistor, not a direct short. This connection isolates the inverting input capacitance from the output pin and improves the frequency response flatness. Usually, for G > 1 V/V applications, the feedback-resistor value must be between 200 Ω and 1.5 kΩ. For values less than 200 Ω, the feedback network presents additional output loading that can degrade the harmonic distortion performance of the OPA690.

A good practice is to target the parallel combination of R_F and R_G (see Figure 7-1) to be less than approximately 300 Ω. The combined impedance R_F || R_G interacts with the inverting input capacitance, placing an additional pole in the feedback network, and thus, a zero in the forward response. Assuming a 2-pF total parasitic on the inverting node, holding R_F || R_G < 300 Ω keeps this pole greater than 250 MHz. Alone, this constraint implies that the feedback resistor R_F can increase to several kΩ at high gains. This result is acceptable as long as the pole formed by R_F and any parasitic capacitance appearing in parallel is kept out of the frequency range of interest.