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SEMICONDUCTOR APPLICATION NOTE
Order this document AN1571/D
Digital Blood Pressure Meter
Prepared C.S. Chua Siew Sensor Application Engineering Singapore,
AN1571
INTRODUCTION
This application note describes Digital Blood Pressure Meter concept which uses integrated pressure sensor, analog signal-conditioning circuitry, microcontroller hardware/software liquid crystal display. sensing system reads cuff pressure (CP) extracts pulses analysis determination systolic diastolic pressure. This design uses integrated pressure sensor (Motorola P/N: MPX5050GP) yielding pressure range mmHg mmHg.
blood pressure (SBP) diastolic blood pressure (DBP) obtained identifying region where there rapid increase then decrease amplitude pulses respectively. Mean arterial pressure (MAP) located point maximum oscillation.
HARDWARE DESCRIPTION OPERATION
cuff pressure sensed Motorola's integrated pressure X-ducerTM. output sensor split into paths different purposes. used cuff pressure while other further processed circuit. Since MPX5050GP signal-conditioned internal op-amp, cuff pressure directly interfaced with analog-to-digital (A/D) converter digitization. other path will filter amplify signal extract amplified version oscillations, which caused expansion subject's each time pressure increases during cardiac systole. output sensor consists signals; oscillation signal riding signal 0.04 Hz). Hence, 2-pole high pass filter designed block signal before amplification oscillation signal. signal properly attenuated, baseline oscillation will constant amplitude each oscillation will have same reference comparison. Figure shows oscillation signal amplifier together with filter.
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CONCEPT OSCILLOMETRIC METHOD
This method employed majority automated non-invasive devices. limb vasculature compressed encircling, inflatable compression cuff. blood pressure reading systolic diastolic blood pressure values read parameter identification point. simplified measurement principle oscillometric method measurement amplitude pressure change cuff cuff inflated from above systolic pressure. amplitude suddenly grows larger pulse breaks through occlusion. This very close systolic pressure. cuff pressure further reduced, pulsation increase amplitude, reaches maximum then diminishes rapidly. index diastolic pressure taken where this rapid transition begins. Therefore, systolic
offset LM324N
0.33u
150k
Figure Oscillation Signal Amplifier
©Motorola Sensor Device Data Motorola, Inc. 1997
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cut-off frequencies approximated following equations. Figure describes frequency response filter. This plot does include gain amplifier.
filter consists networks which determine cut-off frequencies. These poles carefully chosen ensure that oscillation signal distorted lost.
2pR1C1 2pR3C2
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Attenuation (dB)
Oscillation Signal
0.01 Frequency (Hz)
Signal (0.04
Figure Filter Frequency Response
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oscillation signal varies from person person. general, varies from less than mmHg mmHg. From transfer function MPX5050GP, this will translate voltage output signal. Since filter gives attenuation signal, oscillation signal becomes 11.4 respectively. Experiments
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indicate that, amplification factor amplifier chosen that amplified oscillation signal within output limit amplifier Figure 3(a) shows output from pressure sensor Figure 3(b) shows extracted oscillation signal output amplifier.
(volts)
Time (seconds)
Oscillation signal extracted here
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Figure signal output pressure sensor
(volts)
Time (seconds)
Figure Extracted oscillation signal output amplifier
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ratiometric system, variations voltage power supplied system will have effect system accuracy. liquid crystal display (LCD) directly driven from ports microcontroller. operation requires that data backplane (BP) pins must driven alternating signal. This function provided software routine that toggles data backplane approximately rate. Other than LCD, there more devices that connected pulse length converter (PLM) microcontroller; buzzer light emitting diode (LED). buzzer, which connected PLMA, produce different frequencies; 1.953 tones. instance when microcontroller encounters certain error improper inflation cuff, frequency tone alarm. those instance when measurement successful, high frequency pulsation tone will heard. Hence, different musical tone produced differential each condition. addition, used indicate presence heart beat during measurement. microcontroller section system requires certain support hardware allow function. MC34064P-5 provides undervoltage sense function which used reset microprocessor system power-up. crystal provides external portion oscillator function clocking microcontroller provides stable base time based functions, instance calculation pulse rate.
Referring schematic, Figure MPX5050GP pressure sensor connected PORT output amplifier connected PORT microcontroller. This port input on-chip 8-bit analog-to-digital (A/D) converter. pressure sensor provides signal output microprocessor approximately mmHg mmHg applied pressure whereas amplifier provides signal from 0.005 order maximize resolution, separate voltage references should provided instead using supply. this example, input range converter approximately Vdc. This compresses range converter around mmHg mmHg maximize resolution; counts range converter. reference voltage inputs converter. resolution defined following:
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Count [(VXdcr VRL)/(VRH VRL)] count mmHg [(0.2 0)/(3.8 count mmHg [(3.8 0)/(3.8 Therefore resolution counts. This translates system that will resolve 1.24 mmHg. voltage divider consisting connected volts powering system. output pressure sensor ratiometric voltage applied pressure sensor voltage divider connected common supply; this yields system that ratiometric. nature this
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4.7k 4MHz
Regulator MC78L05ACP Input Output 100u 0.33u 100n
Battery
Reset
Input MC34064
4.7k
Buzzer 100R Pressure Sensor MPX5050GP SCLK TCMP1 TCMP2 TCAP1 TCAP2
/RESET /IRQ
Vout
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OSC2 OSC1 LCD5657 330u MC68HC05B16CFN PLMA PLMB PC2/ECLK 0.33u LM324N PD0/AN0 PD1/AN1 PD2/AN2 PD3/AN3 PD4/AN4 PD5/AN5 PD6/AN6 PD7/AN7
Figure Blood Pressure Meter Schematic Drawing
150k
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AN1571
SOFTWARE DESCRIPTION
amplifier. When subroutine TAKE senses decrease continuous duration more than 0.75 seconds, microcontroller will then assume that user longer pumping bulb starts analyze oscillation signal. Figure shows zoom-in view pulse.
Upon system power-up, user needs manually pump cuff pressure approximately mmHg mmHg above previous SBP. During pumping inflation bulb, microcontroller ignores signal output
1.75
(volt)
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Premature pulse
-8.5 -8.3 -8.1 -7.9 -7.7 Time (second) -7.5 -7.3 -7.1
Figure Zoom-in view pulse First all, threshold level valid pulse 1.75 eliminate noise spike. soon amplitude pulse identified, microcontroller will ignore signal prevent false identification presence premature pulse "overshoot" oscillation. Hence, this algorithm only detect pulse rate which less than beats minute. Next, amplitudes pulses detected stored further analysis. microcontroller senses non-typical oscillation envelope shape, error message ("Err") output LCD. user will have exhaust pressure cuff before re-pumping next higher value. algorithm ensures that user exhausts present cuff before allowing re-pumping. Otherwise, venous blood trapped distal affect next measurement. Therefore, user reduce pressure cuff soon possible order recover. Figure flowchart program that controls system.
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MAIN PROGRAM
AN1571
Initialization Clear ports
Display "CAL" output musical tone
Clear variables
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Take amplitude oscillation signal when user stop pumping
Repump?
Calculate also pulse rate
Output high frequency musical tone
there error calculation amplitude envelope detected?
Display "Err"
Display pulse rate. Display "SYS" follow SBP. Display "dlA" follow DBP.
Output frequency alarm
Exhaust cuff before repump
Exhaust cuff before repump
Figure Main program flowchart
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CONCLUSION
This circuit design concept used evaluate Motorola pressure sensors used digital blood pressure meter. This basic circuit easily modified provide suitable output signal level. software also easily modified provide better analysis person.
SELECTION MICROCONTROLLER
Although microcontroller used this project MC68HC05B16, smaller version microcontroller also used. table below shows requirement microcontroller this blood pressure meter design this project. Table Selection microcontroller
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On-chip space kilobytes On-chip space bytes 2-channel converter (min.) 16-bit free running counter timer driver On-chip EEPROM space bytes Power saving Stop Wait modes
reach EUROPE Locations Listed: Motorola Literature Distribution; P.O. 5405, Denver, Colorado 80217. 303-675-2140 1-800-441-2447
REFERENCES
Lucas, Bill (1991). Evaluation System Direct Interface MPX5100 Pressure Sensor with Microprocessor," Motorola Application Note AN1305.
Motorola reserves right make changes without further notice products herein. Motorola makes warranty, representation guarantee regarding suitability products particular purpose, does Motorola assume liability arising application product circuit, specifically disclaims liability, including without limitation consequential incidental damages. "Typical" parameters which provided Motorola data sheets and/or specifications vary different applications actual performance vary over time. operating parameters, including "Typicals" must validated each customer application customer's technical experts. Motorola does convey license under patent rights rights others. Motorola products designed, intended, authorized components systems intended surgical implant into body, other applications intended support sustain life, other application which failure Motorola product could create situation where personal injury death occur. Should Buyer purchase Motorola products such unintended unauthorized application, Buyer shall indemnify hold Motorola officers, employees, subsidiaries, affiliates, distributors harmless against claims, costs, damages, expenses, reasonable attorney fees arising directly indirectly, claim personal injury death associated with such unintended unauthorized use, even such claim alleges that Motorola negligent regarding design manufacture part. Motorola registered trademarks Motorola, Inc. Motorola, Inc. Equal Opportunity/Affirmative Action Employer. Mfax trademark Motorola, Inc. JAPAN: Nippon Motorola Ltd.; Tatsumi-SPD-JLDC, Seibu-Butsuryu-Center, 3-14-2 Tatsumi Koto-Ku, Tokyo 135, Japan. 81-3-3521-8315
MfaxTM: RMFAX0@email.sps.mot.com TOUCHTONE 602-244-6609 ASIA/PACIFIC: Motorola Semiconductors H.K. Ltd.; Ping Industrial Park, Canada ONLY 1-800-774-1848 Ting Road, N.T., Hong Kong. 852-26629298 INTERNET: http://motorola.com/sps
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