The INA122 is an instrumentation amplifier that measures small differential voltages while simultaneously rejecting larger common-mode voltages. The device offers a low power consumption of 60µA (typical) and is designed for portable applications where sensors measure physical parameters, such as changes in fluid, pressure, temperature, or humidity.

The pressure sensor is made of a piezo-resistive element that can be derived as a classical 4-resistor Wheatstone bridge. Changes in the strain gauge resistance on one leg of the Wheatstone bridge (R + ΔR) induces a differential voltage V_DIFF.

Figure 7-3 shows an example circuit for a pressure sensor application. The signal chain connected to the bridge downstream processes the pressure change and can trigger an alarm.

Figure 7-3 Resistive-Bridge Pressure Sensor

Low-tolerance bridge resistors must be used to minimize the offset and gain errors.

Given that there is only a positive differential voltage applied, this circuit is laid out in single supply mode. The excitation voltage, V_EXT, to the bridge must be precise and stable; otherwise, measurement errors can be introduced.