Some applications require an accurate output voltage swing from 0 V to a positive full-scale voltage. A good single-supply op amp may be able to swing within a few millivolts of single-supply ground, but as the output is driven toward 0 V, the output stage of the amplifier prevents the output from reaching the negative supply rail of the amplifier.
The output of the OPAx363 or OPAx364 can be made to swing to ground, or slightly below, on a single-supply power source. To do so requires use of another resistor and an additional, more negative power supply than the op amp negative supply. A pulldown resistor may be connected between the output and the additional negative supply to pull the output down below the value that the output would otherwise achieve, as shown in Figure 24.
This technique does not work with all op amps. The output stage of the OPAx363 and OPA3x64 allows the output voltage to be pulled below that of most op amps, if approximately 500 µA is maintained through the output stage. To calculate the appropriate value load resistor and negative supply, RL = –V / 500 µA. The OPAx363 and OPAx364 are characterized to perform well under the described conditions, maintaining excellent accuracy down to 0 V and as low as –10 mV. Limiting and nonlinearity occur below –10 mV, with linearity returning as the output is again driven above –10 mV.