The open-loop-gain (A_OL) for an op amp is defined as the change in output over the change in the differential input of the op amp. For an ideal op amp the differential input is considered to be zero and the A_OL is infinite. For a practical op amp, however, the differential input is the input offset voltage (V_OS) and the open loop gain is typically on the order of 1 MV/V or 120 dB. The equation for A_OL is A_OL = ΔV_OUT/ ΔV_OS or 20log(ΔV_OUT/ ΔV_OS) in decibels. Rearranging the equation shows that V_OS changes when the output signal changes by ΔV_OS = ΔV_OUT/A_OL. Figure 3-1 shows an inverting amplifier in a gain of –1 V/V where the output swings from -12 V to +12 V. Because the inverting configuration is used the common mode voltage (V_CM) is held constant at 0 V. Changing common mode also causes a shift in offset, so the inverting configuration is useful in illustrating the effect of A_OL on offset without compounding the effect with Vcm.

The specifications for the OPA210 example are shown in Table 3-1. Based on the specification, you would expect the typical offset to be ±5 µV. Notice the test condition listed at the top of the table indicates that the parameters all assume V_CM = V_OUT = midsupply. However, the output voltage ranges from -12 V to +12 V, and the common mode is at 0 V. Thus, the 24 V change in output voltage will result in a corresponding 6 µV change in offset (see calculation Equation 30, and simulation Figure 3-2). In the simulation notice that V_OS = 5 µV specification when V_OUT = 0 V as expected based on the test condition.

Figure 3-1 V_OS Shift Due to Finite A_OL on Inverting OPA210 Configuration

Figure 3-2 Simulated V_OS and Output Swing for Inverting OPA210 Circuit