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. AN1571 2 Sensors Freescale Semiconductor 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 AN1571 Sensors Freescale Semiconductor 3 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. AN1571 4 Sensors Freescale Semiconductor 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 Sensors Freescale Semiconductor +5.0 V 1 Reset Input MC34064 GND 2 3 R9 15k R6 Figure 5. Blood Pressure Meter Schematic Drawing AN1571 5 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. AN1571 6 Sensors Freescale Semiconductor 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 AN1571 Sensors Freescale Semiconductor 7 How to Reach Us: Home Page: www.freescale.com E-mail: [email protected] USA/Europe or Locations Not Listed: Freescale Semiconductor Technical Information Center, CH370 1300 N. Alma School Road Chandler, Arizona 85224 +1-800-521-6274 or +1-480-768-2130 [email protected] Europe, Middle East, and Africa: Freescale Halbleiter Deutschland GmbH Technical Information Center Schatzbogen 7 81829 Muenchen, Germany +44 1296 380 456 (English) +46 8 52200080 (English) +49 89 92103 559 (German) +33 1 69 35 48 48 (French) [email protected] Japan: Freescale Semiconductor Japan Ltd. Headquarters ARCO Tower 15F 1-8-1, Shimo-Meguro, Meguro-ku, Tokyo 153-0064 Japan 0120 191014 or +81 3 5437 9125 [email protected] Asia/Pacific: Freescale Semiconductor Hong Kong Ltd. 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