Digital Blood Pressure Meter

1Motorola Sensor Device Data

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Prepared by: C.S. Chua and Siew Mun HinSensor Application EngineeringSingapore, A/P

INTRODUCTION

This application note describes a Digital Blood PressureMeter concept which uses an integrated pressure sensor,analog signal–conditioning circuitry, microcontrollerhardware/software and a liquid crystal display. The sensingsystem reads the cuff pressure (CP) and extracts the pulsesfor analysis and determination of systolic and diastolicpressure. This design uses a 50 kPa integrated pressuresensor (Motorola P/N: MPX5050GP) yielding a pressurerange of 0 mmHg to 300 mmHg.

CONCEPT OF OSCILLOMETRIC METHOD

This method is employed by the majority of automatednon–invasive devices. A limb and its vasculature arecompressed by an encircling, inflatable compression cuff. Theblood pressure reading for systolic and diastolic bloodpressure values are read at the parameter identification point.

The simplified measurement principle of the oscillometricmethod is a measurement of the amplitude of pressurechange in the cuff as the cuff is inflated from above the systolicpressure. The amplitude suddenly grows larger as the pulsebreaks through the occlusion. This is very close to systolicpressure. As the cuff pressure is further reduced, the pulsationincrease in amplitude, reaches a maximum and thendiminishes rapidly. The index of diastolic pressure is takenwhere this rapid transition begins. Therefore, the systolic

blood pressure (SBP) and diastolic blood pressure (DBP) areobtained by identifying the region where there is a rapidincrease then decrease in the amplitude of the pulsesrespectively. Mean arterial pressure (MAP) is located at thepoint of maximum oscillation.

HARDWARE DESCRIPTION AND OPERATION

The cuff pressure is sensed by Motorola’s integratedpressure X–ducer . The output of the sensor is split into twopaths for two different purposes. One is used as the cuffpressure while the other is further processed by a circuit.Since MPX5050GP is signal–conditioned by its internalop–amp, the cuff pressure can be directly interfaced with ananalog–to–digital (A/D) converter for digitization. The otherpath will filter and amplify the raw CP signal to extract anamplified version of the CP oscillations, which are caused bythe expansion of the subject’s arm each time pressure in thearm increases during cardiac systole.

The output of the sensor consists of two signals; theoscillation signal ( ≈ 1 Hz) riding on the CP signal ( ≤ 0.04 Hz).Hence, a 2–pole high pass filter is designed to block the CPsignal before the amplification of the oscillation signal. If theCP signal is not properly attenuated, the baseline of theoscillation will not be constant and the amplitude of eachoscillation will not have the same reference for comparison.Figure 1 shows the oscillation signal amplifier together withthe filter.

Figure 1. Oscillation Signal Amplifier

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Order this documentby AN1571/D

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Motorola, Inc. 1997

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2 Motorola Sensor Device Data

The filter consists of two RC networks which determine twocut–off frequencies. These two poles are carefully chosen toensure that the oscillation signal is not distorted or lost. The

two cut–off frequencies can be approximated by the followingequations. Figure 2 describes the frequency response of thefilter. This plot does not include the gain of the amplifier.

10

0

–10

–20

–30

–40

–50

–60

–70

–8010 10010.10.01

Frequency (Hz)

Figure 2. Filter Frequency Response

Oscillation Signal (1 Hz)

CP Signal (0.04 Hz)

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The oscillation signal varies from person to person. Ingeneral, it varies from less than 1 mmHg to 3 mmHg. From thetransfer function of MPX5050GP, this will translate to a voltageoutput of 12 mV to 36 mV signal. Since the filter gives anattenuation of 10 dB to the 1 Hz signal, the oscillation signalbecomes 3.8 mV to 11.4 mV respectively. Experiments

indicate that, the amplification factor of the amplifier is chosento be 150 so that the amplified oscillation signal is within theoutput limit of the amplifier (5 mV to 3.5 V). Figure 3(a) showsthe output from the pressure sensor and Figure 3(b) shows theextracted oscillation signal at the output of the amplifier.

Figure 3. CP signal at the output of the pressure sensor

Oscillation signal is extracted here

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Time (seconds)

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Time (seconds)

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Figure 3b. Extracted oscillation signal at the output of amplifier

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Referring to the schematic, Figure 4, the MPX5050GPpressure sensor is connected to PORT D bit 5 and the outputof the amplifier is connected to PORT D bit 6 of themicrocontroller. This port is an input to the on–chip 8–bitanalog–to–digital (A/D) converter. The pressure sensorprovides a signal output to the microprocessor ofapproximately 0.2 Vdc at 0 mmHg to 4.7 Vdc at 375 mmHg ofapplied pressure whereas the amplifier provides a signal from0.005 V to 3.5 V. In order to maximize the resolution, separatevoltage references should be provided for the A/D instead ofusing the 5 V supply. In this example, the input range of the A/Dconverter is set at approximately 0 Vdc to 3.8 Vdc. Thiscompresses the range of the A/D converter around 0 mmHgto 300 mmHg to maximize the resolution; 0 to 255 counts is therange of the A/D converter. VRH and VRL are the referencevoltage inputs to the A/D converter. The resolution is definedby the following:

Count = [(VXdcr – VRL)/(VRH – VRL)] x 255

The count at 0 mmHg = [(0.2 – 0)/(3.8 – 0)] x 255 ≈ 14The count at 300 mmHg = [(3.8 – 0)/(3.8 – 0)] x 255 ≈ 255Therefore the resolution = 255 – 14 = 241 counts. This

translates to a system that will resolve to 1.24 mmHg.The voltage divider consisting of R5 and R6 is connected to

the +5 volts powering the system. The output of the pressuresensor is ratiometric to the voltage applied to it. The pressuresensor and the voltage divider are connected to a commonsupply; this yields a system that is ratiometric. By nature of this

ratiometric system, variations in the voltage of the powersupplied to the system will have no effect on the systemaccuracy.

The liquid crystal display (LCD) is directly driven from I/Oports A, B, and C on the microcontroller. The operation of aLCD requires that the data and backplane (BP) pins must bedriven by an alternating signal. This function is provided by asoftware routine that toggles the data and backplane atapproximately a 30 Hz rate.

Other than the LCD, there are two more I/O devices that areconnected to the pulse length converter (PLM) of themicrocontroller; a buzzer and a light emitting diode (LED). Thebuzzer, which connected to the PLMA, can produce twodifferent frequencies; 122 Hz and 1.953 kHz tones. Forinstance when the microcontroller encounters certain errordue to improper inflation of cuff, a low frequency tone is alarm.In those instance when the measurement is successful, a highfrequency pulsation tone will be heard. Hence, differentmusical tone can be produced to differential each condition. Inaddition, the LED is used to indicate the presence of a heartbeat during the measurement.

The microcontroller section of the system requires certainsupport hardware to allow it to function. The MC34064P–5provides an undervoltage sense function which is used toreset the microprocessor at system power–up. The 4 MHzcrystal provides the external portion of the oscillator functionfor clocking the microcontroller and provides a stable base fortime based functions, for instance calculation of pulse rate.

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Figure 4. Blood Pressure Meter Schematic Drawing

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SOFTWARE DESCRIPTIONUpon system power–up, the user needs to manually pump

the cuff pressure to approximately 160 mmHg or 30 mmHgabove the previous SBP. During the pumping of the inflationbulb, the microcontroller ignores the signal at the output of the

amplifier. When the subroutine TAKE senses a decrease inCP for a continuous duration of more than 0.75 seconds, themicrocontroller will then assume that the user is no longerpumping the bulb and starts to analyze the oscillation signal.Figure 5 shows zoom–in view of a pulse.

–7.1–7.3–7.5–7.7–7.9–8.1–8.3–8.5

Time (second)

Figure 5. Zoom–in view of a pulse

Vo (v

olt)

450 ms

Premature pulse

1.75

First of all, the threshold level of a valid pulse is set to be 1.75V to eliminate noise or spike. As soon as the amplitude of apulse is identified, the microcontroller will ignore the signal for450 ms to prevent any false identification due to the presenceof premature pulse ”overshoot” due to oscillation. Hence, thisalgorithm can only detect pulse rate which is less than 133beats per minute. Next, the amplitudes of all the pulsesdetected are stored in the RAM for further analysis. If themicrocontroller senses a non–typical oscillation envelope

shape, an error message (“Err”) is output to the LCD. The userwill have to exhaust all the pressure in the cuff beforere–pumping the CP to the next higher value. The algorithmensures that the user exhausts all the air present in the cuffbefore allowing any re–pumping. Otherwise, the venous bloodtrapped in the distal arm may affect the next measurement.Therefore, the user has to reduce the pressure in the cuff assoon as possible in order for the arm to recover. Figure 6 is aflowchart for the program that controls the system.

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Figure 6. Main program flowchart

MAIN PROGRAM

InitializationClear I/O ports

Display ”CAL” andoutput a musical tone

Clear all the variables

Take in the amplitude of all theoscillation signal when the

user has stop pumping

Calculate the SBP and DBPand also the pulse rate

Repump?

Is there any errorin the calculation or the

amplitude envelopedetected?

Output a highfrequency

musical tone

Exhaust cuffbefore repump

Exhaust cuffbefore repump

Display pulse rate.Display ”SYS” follow by SBP.Display ”dlA” follow by DBP.

Display ”Err”

Output a lowfrequency

alarm

N

N

N N

Y

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Y Y

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SELECTION OF MICROCONTROLLERAlthough the microcontroller used in this project is

MC68HC05B16, a smaller ROM version microcontroller canalso be used. The table below shows the requirement ofmicrocontroller for this blood pressure meter design in thisproject.

Table 1. Selection of microcontroller

ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ

On–chip ROM space 2 kilobytesÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ

On–chip RAM space 150 bytesÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ2–channel A/D converter (min.)ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ16–bit free running counter timerÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ

LCD driverÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ

On–chip EEPROM space 32 bytes

ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ

Power saving Stop and Wait modes

CONCLUSION

This circuit design concept may be used to evaluateMotorola pressure sensors used in the digital blood pressuremeter. This basic circuit may be easily modified to providesuitable output signal level. The software may also be easilymodified to provide better analysis of the SBP and DBP of aperson.

REFERENCES

Lucas, Bill (1991). “An Evaluation System for Direct Interfaceof the MPX5100 Pressure Sensor with a Microprocessor,”Motorola Application Note AN1305.

Motorola reserves the right to make changes without further notice to any products herein. Motorola makes no warranty, representation or guarantee regardingthe suitability of its products for any particular purpose, nor does Motorola assume any liability arising out of the application or use of any product or circuit, andspecifically disclaims any and all liability, including without limitation consequential or incidental damages. “Typical” parameters which may be provided in Motoroladata sheets and/or specifications can and do vary in different applications and actual performance may vary over time. All operating parameters, including “Typicals”must be validated for each customer application by customer’s technical experts. Motorola does not convey any license under its patent rights nor the rights ofothers. Motorola products are not designed, intended, or authorized for use as components in systems intended for surgical implant into the body, or otherapplications intended to support or sustain life, or for any other application in which the failure of the Motorola product could create a situation where personal injuryor death may occur. Should Buyer purchase or use Motorola products for any such unintended or unauthorized application, Buyer shall indemnify and hold Motorolaand its officers, employees, subsidiaries, affiliates, and distributors harmless against all claims, costs, damages, and expenses, and reasonable attorney feesarising out of, directly or indirectly, any claim of personal injury or death associated with such unintended or unauthorized use, even if such claim alleges thatMotorola was negligent regarding the design or manufacture of the part. Motorola and are registered trademarks of Motorola, Inc. Motorola, Inc. is an EqualOpportunity/Affirmative Action Employer.

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Digital Blood Pressure Meter