3–355
Motorola Sensor Device Data
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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
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.
circuitry,
microcontroller
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
C2
+5V
R2
–
+
+DC offset
U1a
3
2
1
150k
LM324N
0.33u
V
R
1
1
4
V
R
1
C
3
SEMICONDUCTOR APPLICATION NOTE
F
Freescale Semiconductor, Inc.
n
.