The output voltage of the INA351A is developed with respect to the voltage on the pin A_OUT. The voltage on the A_OUT pin sets the common-mode voltage of the instrumentation amplifier OUT pin. In single-supply operation with a bipolar input, setting the output common-mode to a precise mid-supply level is useful and required (for example, 2.75-V in a 5.5-V supply environment) to allow the output signal to swing negative and positive in equal proportions. Traditionally, this is accomplished using a resistive divider from supply and an external reference buffer.

In INA351A, the reference amplifier is integrated on-chip and it is sufficient to only have the resistive divider from supply. With that, the reference amplifier can be connected in G = 1 as shown in Figure 9-1 to provide buffered reference voltage internally to the INA as well as to additional circuits for external use.

Moreover, INA351A has a provision to include servo loop based calibration for external DC offset, drift from the sensor as well as the internal INA offset, drift. This is accomplished using the internal reference amplifier and the three external passive components (R₁, R₂, and C₂) as shown in Figure 9-2. The ratio of resistors R₂ with respect to R₁ sets the gain in the calibration loop to attenuate the DC errors at lower frequencies around DC. At higher frequencies, the capacitor, C₂ shorts R₂ to put the reference buffer in G = 1. This enables the instrumentation amplifier's input differential voltage to influence the INA output at higher frequencies while the reference amplifier in servo loop influences the INA output at lower frequencies around DC. Care should be taken to limit the gain in servo loop so as to maintain sufficient stability. Also, the value of C₂, R₂ should be chosen based on the frequency of input signal at the INA input. Now, the resulting residual DC error after calibration would be that of the reference amplifier itself which is minor in comparison to higher DC offset from the instrumentation amplifier, sensor.

Figure 9-1 INA351A in Reference Buffer Configuration

Figure 9-2 INA351A in Servo-Loop Calibration Configuration