The linear range of the output stage is limited in how close the output voltage can approach ground under zero input conditions. Resulting from an internal node limitation when the REF terminal is grounded (unidirectional configuration) the device gain setting determines how close to ground the device output voltage can achieve when no signal is applied; see Figure 6-14. To overcome this internal node limitation, a small reference voltage (approximately 10 mV) can be applied to the REF terminal to bias the output voltage above this voltage level. The device output swing capability returns to the 10-mV saturation level with this small reference voltage present.

At the lowest gain setting, 25 V/V, the device is capable of accurately measuring input signals that result in output voltages below this 10-mV saturation level of the output stage. For these gain settings, a reference voltage can be applied to bias the output voltage above this lower saturation level to allow the device to monitor these smaller input signals. To avoid common-mode rejection errors, buffer the reference voltage connected to the REF terminal.

A less frequently-used output biasing method is to connect the REF terminal to the supply voltage, V_S. This method results in the output voltage saturating at 200 mV below the supply voltage when no differential input signal is present. This method is similar to the output saturated low condition with no input signal when the REF terminal is connected to ground. The output voltage in this configuration only responds to negative currents that develop negative differential input voltage relative to the device IN– terminal. Under these conditions, when the differential input signal increases negatively, the output voltage moves downward from the saturated supply voltage. The voltage applied to the REF terminal must not exceed the device supply voltage.