7.3.3 Output Voltage Range

The OPAx837 is a rail-to-rail output op amp. Rail-to-rail output typically means that the output voltage swings to within 100 mV of the supply rails. There are two different ways to specify this feature: one is with the output still in linear operation and another is with the output saturated. Saturated output voltages are closer to the power-supply rails than the linear outputs, but the signal is not a linear representation of the input. Saturation and linear operation limits are affected by the output current, where higher currents lead to more voltage loss in the output transistors; see Figure 55.

The Electrical Characteristics tables list saturated output voltage specifications with a 2-kΩ load. Figure 55 illustrates the saturated voltage-swing limits versus output load resistance, and Figure 56 illustrates the output saturation voltage versus load current. Given a light load, the output voltage limits have nearly constant headroom to the power rails and track the power-supply voltages. For example, with a 2-kΩ load and a single
5-V supply, the linear output voltage ranges from 0.10 V to 4.9 V and ranges from 0.1 V to 2.6 V for a 2.7-V supply. The delta from each power-supply rail is the same in either case: 0.1 V.

With devices like the OPA837 and OPA2837 where the input range is lower than the output range, typically the input limits the available signal swing only in a noninverting gain of 1 V/V. Signal swing in noninverting configurations in gains greater than +1 V/V and inverting configurations in any gain are typically limited by the output voltage limits of the op amp.