34.4.4 Differential Input

The Op Amp supports rail-to-rail operation with both PMOS and NMOS differential pairs. Each pair can be disabled to facilitate independent calibration, by setting the DIFFCON[1:0] bits in the AMPxCON1 register.

The complementary pair inputs allow the Common mode voltage to extend to the supply rails without phase inversion. These are used to design the input stage of single-supply, voltage-feedback amplifiers.

In the region near V_SS, the offset error of the PMOS portion of the input stage is dominant, and in the region near V_DD, the offset error is dominated by the NMOS transistor pair. That is, as the input common mode voltage approaches V_DD, the PMOS transistors go into the cutoff region and the input transitions to the NMOS transistors, which enables operation above the positive supply. With a complementary pair input stage, the PMOS transistors enable operation slightly below the negative supply and the NMOS transistors allow for operation slightly above the positive supply.

With the topology used, the amplifier effectively combines the advantages of the PMOS and NMOS transistors for true rail-to-rail input operation. If the input range is known to exclude voltages around the negative rail, then the PMOS transistors can be disabled to keep the Op Amp offset more constant and dependent only on the NMOS input pair. Similarly, if the input range never includes voltages around the positive rail, then the NMOS transistors can be disabled to keep the Op Amp offset more constant and dependent only on the PMOS input pair. This combination of the PMOS and NMOS transistors allows the users to trade input voltage range for improved INL and offset voltage operation.