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

C o n t e n t s

ICs for Wireless ECG Holter Monitors >>

Blood Analysis Systems: Impedance Measurement >>

Blood Analysis Systems: Flow Cytometry >>

Digital X-Ray Technology >>

Home Glucose Monitoring Using Amperometry >>

Biopotential Electrode Sensing in ECG, EMG, and EEG Monitors >>

Photodiode Sensing in Pulse Oximeters >>

Oscillometric Blood Pressure Measurement >>

Using Thermopile Sensors in IR Digital Thermometers Measurement >>

IEC 60601-1-Compliant Digital Isolators Measurement >>

Switches and Multiplexers Offer Industry-Leading Performance in Small Packaging >>

Power Distribution in Imaging Systems Measurement >>

Piezoresistive Sensing ICs In Portable Infusion Pumps Measurement >>

AVNET

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Analog is Everywhere

Analog Devices
Piezoresistive Sensing ICs in Portable Infusion Pumps

Infusion pumps administer fluids into a patient’s 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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