MPXM2102AS datasheet(501/670 Pages) MOTOROLA

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Motorola Sensor Device Data

www.motorola.com/semiconductors

AN1571

Digital Blood Pressure Meter

Prepared by: C.S. Chua and Siew Mun Hin

Sensor Application Engineering

Singapore, A/P

INTRODUCTION

This application note describes a Digital Blood Pressure

Meter concept which uses an integrated pressure sensor,

analog

signal–conditioning

circuitry,

microcontroller

hardware/software and a liquid crystal display. The sensing

system reads the cuff pressure (CP) and extracts the pulses

for analysis and determination of systolic and diastolic

pressure. This design uses a 50 kPa integrated pressure

sensor (Motorola P/N: MPXV5050GP) yielding a pressure

range of 0 mmHg to 300 mmHg.

CONCEPT OF OSCILLOMETRIC METHOD

This method is employed by the majority of automated

non–invasive devices. A limb and its vasculature are

compressed by an encircling, inflatable compression cuff. The

blood pressure reading for systolic and diastolic blood

pressure values are read at the parameter identification point.

The simplified measurement principle of the oscillometric

method is a measurement of the amplitude of pressure

change in the cuff as the cuff is inflated from above the systolic

pressure. The amplitude suddenly grows larger as the pulse

breaks through the occlusion. This is very close to systolic

pressure. As the cuff pressure is further reduced, the pulsation

increase in amplitude, reaches a maximum and then

diminishes rapidly. The index of diastolic pressure is taken

where this rapid transition begins. Therefore, the systolic

blood pressure (SBP) and diastolic blood pressure (DBP) are

obtained by identifying the region where there is a rapid

increase then decrease in the amplitude of the pulses

respectively. Mean arterial pressure (MAP) is located at the

point of maximum oscillation.

HARDWARE DESCRIPTION AND OPERATION

The

cuff pressure is sensed by Motorola’s integrated

pressure X–ducer

™. The output of the sensor is split into two

paths for two different purposes. One is used as the cuff

pressure while the other is further processed by a circuit.

Since MPXV5050GP is signal–conditioned by its internal

op–amp, the cuff pressure can be directly interfaced with an

analog–to–digital (A/D) converter for digitization. The other

path will filter and amplify the raw CP signal to extract an

amplified version of the CP oscillations, which are caused by

the expansion of the subject’s arm each time pressure in the

arm increases during cardiac systole.

The output of the sensor consists of two signals; the

oscillation signal (

≈ 1 Hz) riding on the CP signal ( ≤ 0.04 Hz).

Hence, a 2–pole high pass filter is designed to block the CP

signal before the amplification of the oscillation signal. If the

CP signal is not properly attenuated, the baseline of the

oscillation will not be constant and the amplitude of each

oscillation will not have the same reference for comparison.

Figure 1 shows the oscillation signal amplifier together with

the filter.

Figure 1. Oscillation Signal Amplifier

+5V

–

+DC offset

U1a

150k

LM324N

0.33u

MOTOROLA

SEMICONDUCTOR APPLICATION NOTE

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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