7.3.5 Common-Mode Voltage Range

The OPA2156 is a 36-V, true rail-to-rail input 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+) – 2.25 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+) – 1.25 V. There is a small transition region, typically (V+) – 2.25V to (V+) – 1.25 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.

To achieve the best performance for two-stage rail-to-rail input amplifiers, avoid the transition region when possible. The OPA2156 uses a precision trim for both the N-channel and P-channel regions. This technique enables significantly lower levels of offset than previous-generation devices, causing variance in the transition region of the input stages to appear exaggerated relative to offset over the full common-mode range.