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SEMICONDUCTOR APPLICATION NOTE

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MOTOROLA
SEMICONDUCTOR APPLICATION NOTE
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Digital Blood Pressure Meter
Prepared by: C.S. Chua and Siew Mun Hin Sensor Application Engineering Singapore, A / P
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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: MPX5050GP) yielding a pressure range of 0 mmHg to 300 mmHg.
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
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
+DC offset +5V R3 1M 11 3 2 + - 4 U1a 1 LM324N
C2 Vi 0.33u
R2 R1 150k 1k
Figure 1. Oscillation Signal Amplifier
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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 two cut-off frequencies can be approximated by the following equations. Figure 2 describes the frequency response of the filter. This plot does not include the gain of the amplifier.
1 2pR1C1 1 2pR3C2
10 0 -10 -20 Attenuation (dB)
Oscillation Signal (1 Hz)
-30 -40 -50 -60 -70 -80 0.01 0.1 1 Frequency (Hz) 10 100
CP Signal (0.04 Hz)
Figure 2. Filter Frequency Response
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The oscillation signal varies from person to person. In general, it varies from less than 1 mmHg to 3 mmHg. From the transfer function of MPX5050GP, 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 mV to 3.5 V). Figure 3(a) shows the output from the pressure sensor and Figure 3(b) shows the extracted oscillation signal at the output of the amplifier.
2.5 2 Vi (volts)
1.5 1 0.5 0 0 5 10 15 20 Time (seconds)
Oscillation signal is extracted here
Figure 3. CP signal at the output of the pressure sensor
MAP SBP
2 Vo (volts)
0 10 15 20 Time (seconds) 25 30 35
Figure 3b. Extracted oscillation signal at the output of amplifier
Motorola Sensor Device Data
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Motorola Sensor Device Data
R10 10M +5V X1 4.7k 4MHz C3 C4 1 R9 1 +5V +5V
5V Regulator MC78L05ACP 3 Input Output C5 GND 2 22p C6 C8 100u 22p 0.33u 100n +5V
9V Battery
Reset 3
2 Input GND MC34064
4.7k + 36R
Buzzer 100R RD Pressure Sensor MPX5050GP 1 +5V 52 TDO 51 SCLK 2 TCMP1 1 TCMP2 TCAP1 TCAP2 22 23 50
18 19 +5V
Vs LED Vout GND 2
Motorola Sensor Device Data
17 OSC2 VDD OSC1 10 +5V 16 16 23 DP1 LCD5657 G1 22 F1 21 A1 20 B1 19 VRH VRL 8 7 4 C1 DP 18 D1 17 E1 3 37 G4 36 F4 35 A4 34 B4 7 C4 6 D4 E4 5 L DP 12 27 26 2 DP2 G2 F2 DP 1 C 14 D2 13 E2 330u C7 MC68HC05B16CFN +5V E G B F A DP3 G3 F3 A3 B3 C3 D3 E3 L L BP BP 28 40 1 8 32 31 30 29 11 10 9 R4 24k +5V 20 PLMA 21 PLMB 49 PC0 48 PC1 47 PC2 / ECLK 46 PC3 45 PC4 44 PC5 43 PC6 42 PC7 PA0 PA1 PA2 PA3 PA4 PA5 PA6 PA7 31 30 29 28 27 26 25 24 C2 3 0.33u LM324N 2 1 14 13 12 11 9 5 4 3 PD0 / AN0 PD1 / AN1 PD2 / AN2 PD3 / AN3 PD4 / AN4 PD5 / AN5 PD6 / AN6 PD7 / AN7 PB0 PB1 PB2 PB3 PB4 PB5 PB6 PB7 39 38 37 36 35 34 33 32 25 A2 D 24 B2 15 C2
Figure 4. Blood Pressure Meter Schematic Drawing
R2 150k
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SOFTWARE DESCRIPTION
Upon system power-up, the user needs to manually pump the cuff pressure to approximately 160 mmHg or 30 mmHg 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 5 shows zoom-in view of a pulse.
450 ms 1.75
Vo (volt)
Premature pulse
-8.5 -8.3 -8.1 -7.9 -7.7 Time (second) -7.5 -7.3 -7.1
Figure 5. 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 re-pumping 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 6 is a flowchart for the program that controls the system.
Motorola Sensor Device Data
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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
Calculate the SBP and DBP and also the pulse rate
Output a high frequency musical tone
Display "Err"
Display pulse rate. Display "SYS" follow by SBP. Display "dlA" follow by DBP.
Output a low frequency alarm
Exhaust cuff before repump
Figure 6. Main program flowchart
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SELECTION OF MICROCONTROLLER
Although the microcontroller used in this project is MC68HC05B16, a smaller ROM version microcontroller can also be used. The table below shows the requirement of microcontroller for this blood pressure meter design in this project. Table 1. Selection of microcontroller
CONCLUSION
This circuit design concept may be used to evaluate Motorola 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.
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
How to reach us: USA / EUROPE / Locations Not Listed: Motorola Literature Distribution P.O. Box 5405, Denver, Colorado 80217. 303-675-2140 or 1-800-441-2447
REFERENCES
Lucas, Bill (1991). "An Evaluation System for Direct Interface of the MPX5100 Pressure Sensor with a Microprocessor, " Motorola Application Note AN1305.
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