Medical Electronics ICs
Volume 8, Issue 4 YOUR SEMICONDUCTOR SOLUTIONS RESOURCE

Piezoresistive Sensing ICs in Portable Infusion Pumps

Infusion pumps administer fluids into a patientís circulatory system. Doses administered can be as small as 100 µL. Control is achieved by measuring the pressure of the fluid entering the patient through the delivery tube. Since battery life is a critical selling feature in portable infusion pumps, low power components and power-down are desired features.

Solution
Piezoresistive sensors consist of a simple two-contact diffused n-well or p-well, within a p-substrate or n-substrate. An advanced stress sensor derived from piezoresistors is known as a Wheatstone Bridge. This configuration measures an unknown electrical resistance balancing two legs of a bridge circuit, one leg of which includes the unknown component. An excitation voltage or current is applied to the bridge. Pressure applied to the piezoelectric element changes the resistance, thus creating a change in voltage.

The AD627 is an integrated instrumentation amplifier that measures the differential voltage while operating at only 85 µA max. It is well-suited for portable insulin infusion pumps, allowing for longer battery life. The AD627 minimizes errors with low voltage offset, offset drift, gain error, and gain drift. Drive the reference pin of the AD627 with a low impedance voltage source to level shift the output voltage. The output signal appears as the voltage difference between the output pin and the externally applied voltage on the REF pin. The ADR36x family of micropower precision band gap voltage references uses a patented temperature drift curvature correction technique that provides a stable output voltage from a minimum supply of 300 mV above the output. Their advanced design eliminates the need for external capacitors, which further reduces board space and system cost—ideal for battery-operated applications.

In one popular method, the piezoelectric resistive sensor abuts the tube delivering the fluid. Pressure on the tube wall is measured by the sensor.

ADR361 AD627

The piezoresistive element forms a Wheatstone Bridge. Pressure on the sensor causes a proportional change in voltage.

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