The OPA928 is a 16-V, true rail-to-rail input/output operational amplifier with an input common-mode range that extends 100 mV beyond either supply rail. This wide range is achieved with paralleled complementary N-channel and P-channel differential input pairs. The N-channel pair is active for input voltages close to the positive rail, typically (V+) – 3 V to 100 mV above the positive supply. The P-channel pair is active for inputs from 100 mV below the negative supply to approximately (V+) – 1.5 V. There is a small transition region, typically (V+) –3 V to (V+) – 1.5 V in which both input pairs are active. This transition region varies modestly with process variation. Within this region PSRR, CMRR, offset voltage, offset drift, noise, and THD performance are degraded compared to operation outside this region.

The OPA928 uses a precision trim for both the N-channel and P-channel regions enabling significantly lower levels of offset than previous-generation devices. In inverting configurations, such as with a transimpedance amplifier, the common-mode voltage is constant and set by the voltage at the noninverting pin. Therefore, in inverting configurations, the transition region can be easily avoided and is typically not a problem. In noninverting configurations, such as with a buffer, the common-mode voltage can vary widely; take care to avoid the transition region when possible.

The OPA928 guard buffer features the same complementary input stage. The input bias performance of the OPA928 is sensitive to small shifts in offset voltage. The shift in offset in the transition region is presented across the internal protection diodes and causes increased leakage. The increase in leakage can be significant and degrade the input bias performance of the OPA928. Avoid operating in the transistion region when possible to achieve specified input bias current performance.