Some applications require consideration of the limitations of the guard buffer output swing. One such application is the single-supply inverting amplifier with a common-mode voltage equal to the rail, most commonly ground. The guard buffer cannot drive the guard all the way to the rail. The internal guard buffer features a rail-to-rail output stage and is capable of driving the guard to within 15mV of the rail. The small voltage difference manifests as a differential voltage across the antiparallel diodes as shown in Figure 7-3. Even a small differential voltage of 15mV can cause a significant amount of leakage through the diodes at high temperatures.

To avoid the input common-mode limitation of the guard buffer in inverting configurations with zero-common mode voltage, connect the guard pins directly to ground as done in Figure 7-4.

Figure 7-5 shows the input bias current performance of the OPA928 near the negative rail when the guard pins are left floating compared to when the guard pins are tied to ground.

Noninverting, high-gain configurations can be susceptible to similar limitations. The limitation occurs when the input signal is less than the specified output swing of the guard buffer. To circumvent this issue, consider using dual supplies.