Download AN1571, Digital Blood Pressure Meter

Freescale Semiconductor
Application Note
AN1571
Rev 1, 05/2005
Digital Blood Pressure Meter
by: C.S. Chua and Siew Mun Hin, Sensor Application Engineering
Singapore, A/P
INTRODUCTION
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.
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 (Freescale Semiconductor, Inc.P/N: MPXV5050GP)
yielding a pressure range of 0 mm Hg to 300 mm Hg.
HARDWARE DESCRIPTION AND OPERATION
The cuff pressure is sensed by Freescale'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 opamp, 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.
CONCEPT OF OSCILLOMETRIC METHOD
This method is employed by the majority of automated noninvasive 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
+DC Offset
0.33 µ
+
1
LM324N
4
2
U1a
R1
1k
C1
33u
R2
150k
Figure 1. Oscillation Signal Amplifier
© Freescale Semiconductor, Inc., 2005. All rights reserved.
Vo
11
1M
3
Vi
C2
R3
+5.0V
two cut-off frequencies can be approximated by the following
equations. Figure 2describes the frequency response of the
filter. This plot does not include the gain of the amplifier.
The filter consists of two RC networks which determine two
cut-off frequencies. These two poles are carefully chosen to
ensure that the oscillation signal is not distorted or lost. The
1
f
P1 =
2πR1C1
f
P2 =
2πR3C2
1
10
0
-10
Attenuation (dB)
-20
Oscillation Signal (1 Hz)
-30
-40
-50
CP Signal (0.04 Hz)
-60
-70
-80
0.01
0.1
1
10
100
Frequency (Hz)
Figure 2. Filter Frequency
The oscillation signal varies from person to person. In
general, it varies from less than 1 mm Hg to 3 mm Hg. From
the transfer function of MPXV5050GP, this will translate to a
voltage output of 12 mV to 36 mV signal. Since the filter gives
an attenuation of 10 dB to the 1 Hz signal, the oscillation signal
becomes 3.8 mV to 11.4 mV respectively. Experiments
indicate that, the amplification factor of the amplifier is chosen
to be 150 so that the amplified oscillation signal is within the
output limit of the amplifier (5.0 mV to 3.5 V). Figure 3 shows
the output from the pressure sensor and Figure 4 illustrates
the extracted oscillation signal at the output of the amplifier.
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3
2.5
Vi (Volts)
2
1.5
Oscillation signal is extracted here
1
0.5
0
0
5
10
15
20
25
30
35
40
Time (seconds)
Figure 3. CP Signal at the Output of the Pressure Sensor
3.5
MAP
3
SBP
DBP
Vo (Volts)
2.5
2
1.5
1
0.5
0
10
15
20
25
30
35
Time (seconds)
Figure 4. Extracted Oscillation Signal at the Output of Amplifier
Referring to the schematic, Figure 5, the MPX5050GP
pressure sensor is connected to PORT D bit 5 and the output
of the amplifier is connected to PORT D bit 6 of the
microcontroller. This port is an input to the on-chip 8-bit
analog-to-digital (A/D) converter. The pressure sensor
provides a signal output to the microprocessor of
approximately 0.2 Vdc at 0 mm Hg to 4.7 Vdc at 375 mm Hg
of applied pressure whereas the amplifier provides a signal
from 0.005 V to 3.5 V. In order to maximize the resolution,
separate voltage references should be provided for the A/D
instead of using the 5 V supply. In this example, the input
range of the A/D converter is set at approximately 0 Vdc to 3.8
Vdc. This compresses the range of the A/D converter around
0 mm Hg to 300 mm Hg to maximize the resolution; 0 to 255
counts is the range of the A/D converter. VRH and VRL are the
reference voltage inputs to the A/D converter. The resolution
is defined by the following:
Count = [(VXdcr - VRL)/(VRH - VRL)] x 255
The count at 0 mm Hg = [(0.2 - 0)/(3.8 - 0)] x 255 ≈ 14
The count at 300 mm Hg = [(3.8 - 0)/(3.8 - 0)] x 255 ≈ 255
Therefore the resolution = 255 - 14 = 241 counts. This
translates to a system that will resolve to 1.24 mm Hg.
The voltage divider consisting of R5 and R6 is connected to
the +5 volts powering the system. The output of the pressure
sensor is ratiometric to the voltage applied to it. The pressure
sensor and the voltage divider are connected to a common
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supply; this yields a system that is ratiometric. By nature of this
ratiometric system, variations in the voltage of the power
supplied to the system will have no effect on the system
accuracy.
The liquid crystal display (LCD) is directly driven from I/O
ports A, B, and C on the microcontroller. The operation of a
LCD requires that the data and backplane (BP) pins must be
driven by an alternating signal. This function is provided by a
software routine that toggles the data and backplane at
approximately a 30 Hz rate.
Other than the LCD, there are two more I/O devices that are
connected to the pulse length converter (PLM) of the
microcontroller; a buzzer and a light emitting diode (LED). The
buzzer, which connected to the PLMA, can produce two
different frequencies; 122 Hz and 1.953 kHz tones. For
instance when the microcontroller encounters certain error
due to improper inflation of cuff, a low frequency tone is alarm.
In those instance when the measurement is successful, a high
frequency pulsation tone will be heard. Hence, different
musical tone can be produced to differential each condition. In
addition, the LED is used to indicate the presence of a heart
beat during the measurement.
The microcontroller section of the system requires certain
support hardware to allow it to function. The MC34064P-5
provides an undervoltage sense function which is used to
reset the microprocessor at system power-up. The 4 MHz
crystal provides the external portion of the oscillator function
for clocking the microcontroller and provides a stable base for
time based functions, for instance calculation of pulse rate.
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GND
Vs
C5
0.33u
Vout
1
Pressure Sensor
MPXV5050GP
+5.0 V
3
2
0.33u
C2
2
24k
+5.0 V
R4
GND
3 Input Output 1
R3
MC78L05ACP
+5.0 V
2
3
1M
5.0 V Regulator
10k
R0
R1
C1
1k
33u
+5.0 V
LM324N
R2
150k
C8
1
Buzzer
100n
C7
C6
330u
100R
100u
C3
VDD
OSC2
22p
X1
22p
PD0/AN0
PD1/AN1
PD2/AN2
PD3/AN3
PD4/AN4
PD5/AN5
PD6/AN6
PD7/AN7
14
13
12
11
9
5
4
3
+5.0 V
MC68HC05B16CFN
PB0
PB1
PB2
PB3
PB4
PB5
PB6
PB7
VRH
PLMA
VRL
PLMB
PA0
PC0
PA1
PC1
PC2/ECLK PA2
PA3
PC3
PA4
PC4
PA5
PC5
PA6
PC6
PA7
PC7
20
21
49
48
47
46
45
44
43
42
RD
TCAP1
TCAP2
/RESET
/IRQ
OSC1
4MHz
2
TCMP1
1 TCMP2
52 TDO
51 SCLK
10
17
R10
10M
39
38
37
36
35
34
33
32
8
7
31
30
29
28
27
26
25
24
50
22
23
18
19
+5.0 V
C4
16
+5.0 V
+5.0 V
R5
11
4.7k + 36R
4
R8
4.7k
LED
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+5.0 V
1 Reset Input
MC34064
GND
2
3
R9
15k
R6
Figure 5. Blood Pressure Meter Schematic Drawing
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9.0 V Battery
12
27
26
25
24
15
14
13
16
23
22
21
20
19
18
17
2
DP2
G2
F2 DP E
A2
D 1
B2
C2
C
D2
E2
DP
L
B
G
L
LCD5657
DP1
G1
F1
4
A1
B1
C1 DP
D1
3
E1
F
5
34
7
6
37
36
35
28
L
40
BP
1
BP
8
DP3
32
G3
31
F3
30
A3
A
29
B3
11
C3
10
D3
9
E3
G4
F4
A4
B4
C4
D4
E4
SOFTWARE DESCRIPTION
Upon system power-up, the user needs to manually pump
the cuff pressure to approximately 160 mm Hg or 30 mm Hg
above the previous SBP. During the pumping of the inflation
bulb, the microcontroller ignores the signal at the output of the
amplifier. When the subroutine TAKE senses a decrease in
CP for a continuous duration of more than 0.75 seconds, the
microcontroller will then assume that the user is no longer
pumping the bulb and starts to analyze the oscillation signal.
Figure 6 shows zoom-in view of a pulse.
VO (volt)
450 ms
1.75
Premature Pulse
-8.5
-8.3
-8.1
-7.9
-7.7
-7.5
-7.3
-7.1
Time (second)
Figure 6. Zoom-In View of a Pulse
First of all, the threshold level of a valid pulse is set to be
1.75 V to eliminate noise or spike. As soon as the amplitude
of a pulse is identified, the microcontroller will ignore the signal
for 450 ms to prevent any false identification due to the
presence of premature pulse "overshoot" due to oscillation.
Hence, this algorithm can only detect pulse rate which is less
than 133 beats per minute. Next, the amplitudes of all the
pulses detected are stored in the RAM for further analysis. If
the microcontroller senses a non-typical oscillation envelope
shape, an error message (“Err”) is output to the LCD. The user
will have to exhaust all the pressure in the cuff before repumping the CP to the next higher value. The algorithm
ensures that the user exhausts all the air present in the cuff
before allowing any re-pumping. Otherwise, the venous blood
trapped in the distal arm may affect the next measurement.
Therefore, the user has to reduce the pressure in the cuff as
soon as possible in order for the arm to recover. Figure 7 on
the following page is a flowchart for the program that controls
the system.
SELECTION OF MICROCONTROLLER
Although the microcontroller used in this project is
MC68HC05B16, a smaller ROM version microcontroller can
also be used. The list below shows the requirement of
microcontroller for this blood pressure meter design in this
project.
• On-chip ROM space: 2 kilobytes
• On-chip RAM space: 150 bytes
• 2-channel A/D converter (min.)
• 16-bit free running counter timer
• LCD driver
• On-chip EEPROM space: 32 bytes
• Power saving Stop and Wait modes
CONCLUSION
This circuit design concept may be used to evaluate
Freescale pressure sensors used in the digital blood pressure
meter. This basic circuit may be easily modified to provide
suitable output signal level. The software may also be easily
modified to provide better analysis of the SBP and DBP of a
person.
REFERENCES
Lucas, Bill (1991). “An Evaluation System for Direct
Interface of the MPX5100 Pressure Sensor with a
Microprocessor,” Freescale Application Note AN1305.
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Main Program
Initialization
Clear I/O ports
Display "CAL" and
output a musical tone
Clear all the variables
Take in the amplitude of all the
oscillation signal when the
user has stop pumping
Repump?
Y
N
Calculate the SBP and DBP
and also the pulse rate
Output a high
frequency
musical tone
Display pulse rate.
Display "SYS" follow by SBP.
Display "dlA" follow by DBP.
Y
Is there any error
in the calculation or the
amplitude envelope
detected?
N
N
Y
N
Display "Err"
Output a low
frequency alarm
Exhaust cuff
before repump
Exhaust cuff
before repump
Y
Figure 7. Main Program Flowchart
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AN1571
Rev. 1
05/2005
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