The INA823 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 250 µA (max) and high precision, thus minimizing errors with voltage offset, offset drift and gain error.

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 9-2 showcases an exemplary 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.

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 error is introduced.

The REF5025 is a low-noise, low-drift (3 ppm/C), and high-precision (0.05%) voltage reference that is an excellent option to generate the excitation voltage V_EXT.

The following subsections give the design requirements and detailed design procedure for an application with a occlusion pressure sensor.

For more information and design tips to consider when using a resistive-bride pressure sensor, see the Design tips for a resistive-bridge pressure sensor in industrial process-control systems analog applications journal.