The INA351 is an integrated instrumentation amplifier that measures small differential voltages while simultaneously rejecting larger common-mode voltages. The device offers a low power consumption of 110 µA (typical) and has a smaller form factor.

The device is designed for portable applications where sensors measure physical parameters, such as changes in fluid, pressure, temperature, or humidity. An example of a pressure sensor used in the medical sector is in portable infusion pumps or dialysis machines.

The pressure sensor is made of a piezo-resistive element that can be derived as a classical 4-resistor Wheatstone bridge.

Occlusion (infusion of fluids, medication, or nutrients) happens only in one direction, and therefore can only cause the resistive element (R) to expand. This expansion causes a change in voltage on one leg of the Wheatstone bridge, which induces a differential voltage V_DIFF.

Figure 9-4 shows an example circuit for an occlusion pressure sensor application, as required in infusion pumps. When blockage (occlusion) occurs against a set-point value, the tubing depresses, thus causing the piezo-resistive element to expand (Node AD: R + ΔR). The signal chain connected to the bridge downstream processes the pressure change and can trigger an alarm.

Figure 9-4 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-ended supply mode. The excitation voltage, V_EXT, to the bridge must be precise and stable; otherwise, measurement errors can be introduced.