Certain single-supply applications require the op amp output to swing from 0 V to a positive full-scale voltage and have high accuracy. An example is an op amp employed to drive a single-supply ADC having an input range from 0 V to 5 V. Rail-to-rail output amplifiers with very light output loading can achieve an output level within millivolts of 0 V (or +V_S at the high end), but not 0 V. Furthermore, the deviation from 0 V only becomes greater as the load current required increases. This increased deviation is a result of limitations of the CMOS output stage.

When a pulldown resistor is connected from the amplifier output to a negative voltage source, the OPAx365 can achieve an output level of 0 V, and even a few millivolts below 0 V. Below this limit, nonlinearity and limiting conditions become evident. Figure 8-4 illustrates a circuit using this technique.

A pulldown current of approximately 500 μA is required when the OPAx365 is connected as a unity-gain buffer. A practical termination voltage (V_NEG) is −5 V, but other convenient negative voltages also can be used. Pulldown resistor R_L is calculated from R_L = [(V_O − V_NEG) / (500 μA)].

Using a minimum output voltage (V_O) of 0 V, R_L = [0 V − (−5 V)] / (500 μA)] = 10 kΩ. Keep in mind that lower termination voltages result in smaller pulldown resistors that load the output during positive output voltage excursions.

Keep these precautions in mind when driving a capacitive load because these conditions can affect circuit transient response and stability.

Figure 8-4 Swing-to-Ground