Single-supply amplifiers do not truly swing rail to rail at the output. Near the rail, the amplifier is nonlinear. For linear operation, the output of single-supply amplifiers can come within only 50 to 300 mV of each rail (Figure 1).

Single-supply-amplifier, rail-to-rail-output ads can give a false sense of security. Figure 1 shows a typical single-supply amplifier’s output swing as you drive the output to the rails.

The amplifier’s linearity starts to degrade long before reaching the output-swing maximums, and the amplifier output never reaches either rail.

The conditions of the dc-open-loop-gain specification define the amplifier’s linear operating output range. The dc open loop gain in decibels is 20 log(ΔV_OUT/ΔV_OS), where V_OUT is the output voltage and V_OS is the input offset voltage. When you drive the output high, V_H is the maximum voltage level at the output in the dc-open-loop-gain measurement. V_OH is the absolute maximum voltage level with respect to V_DD (drain-to-drain voltage) that the output can reach. V_L is the minimum voltage level at the output in the dc-open-loop-gain measurement, and V_OL is the absolute minimum voltage level that the output can reach. V_H is less than V_OH, and V_L is greater than V_OL.

From a signal-chain perspective, you can see an op amp’s output limitations to swinging rail to rail when the op amp is driving an ADC. The FFT plot in Figure 2a shows the amplifier/ADC-combination response to a 1-kHz signal in a 5V system. The amplifier’s typical closed-loop bandwidth is about 3 MHz with a typical slew rate of 2.3V/µsec. The amplifier output voltage swings from 140 mV to 4.66V. In this 5V-supply system, the headroom between the signal and rails is 140 mV. For this amplifier, the V_OL minimum specification is 15 mV above ground. The V_OH maximum specification is V_DD–20 mV.

Figure 2a illustrates the nonlinearity-output-stage effects with a single-supply CMOS amplifier by showing distortion at 2, 3, and 4 kHz and so on. By reducing the amplifier’s output signal to 272 mV from each rail, the data looks perfect with only the ADC distortion (Figure 2b).

When using a single-supply amplifier, read the fine print! Some single-supply amps have output-stage charge pumps, allowing the amplifier’s output swing to go to and well beyond the power-supply rails. In every case, read your data sheet and refer to the conditions on the open-loop-gain test.