To achieve a femtoampere-level input bias current, the OPA928 uses an internal, high-precision, rail-to-rail guard buffer connected to the noninverting input. The guard buffer follows the voltage at the input of the OPA928 to generate a near zero differential voltage across the internal antiparallel diodes (detailed in Section 6.3.2), nearly eliminating the leakage current though the diodes. The guard buffer output can be accessed through the guard pins (2 and 7). Use the guard pins to protect external components and input traces from possible current leakage paths. The guard buffer is isolated from large capacitive loads that can be present at the guard pins by a nominal 1kΩ resistor. For more on guarding, see Section 7.1.2.

The guard buffer is a rail-to-rail input and output amplifier with the same complementary input stage as the OPA928. Like all rail-to-rail amplifiers, the guard buffer output cannot swing all the way to the rail by a few millivolts. This is particularly important in some special single-supply cases because the input bias performance of the OPA928 is sensitive to small differential voltages across the internal antiparallel diodes; see also Section 7.1.3.