AN58128.pdf

THIS SPEC IS OBSOLETE
Spec No:
001-58128
Spec Title:
BLOOD PRESSURE MONITOR WITH
PSOC(R) - AN58128
Sunset Owner:
Shruti Hanumanthaiah (SSHH)
Replaced By:
None
AN58128
Blood Pressure Monitor with PSoC®
Author: Sanjeev Kumar K.
Associated Project: Yes
Associated Part Family: CY8C27xxx, CY8C28xxx, CY8C29xxx
®
Software Version: PSoC Designer™ 5.3
Related Application Notes: For a complete list of the application notes, click here.
If you have a question, or need help with this application note, contact the author at
[email protected]
Blood pressure is one of the vital signs in the human body. It is measured using both invasive and non invasive
®
techniques. This application note demonstrates how to build a non invasive blood pressure monitor using the PSoC 1.
This design does not use any external active components to buffer, amplify, and filter the signal.
Contents
Introduction
Introduction .......................................................................1
Operation Principle ............................................................2
Block Diagram ...................................................................2
Pressure Sensor...........................................................3
Amplifier .......................................................................3
Filter .............................................................................4
Multiplexer and ADC ....................................................5
Pneumatics...................................................................5
Display..........................................................................5
Working .............................................................................8
Project Description ............................................................8
Calibration .........................................................................8
Summary ......................................................................... 10
Related Application Notes ............................................... 10
Appendix A ...................................................................... 11
Segment LCD Glass Drive and Capacitive Sensing ... 11
Worldwide Sales and Design Support ............................. 13
The non invasive method of monitoring blood pressure is
widely used. It measures arterial systolic and diastolic
pressures of the human body. This device can also
measure heart rate. Table 1 describes various non invasive
methods used in blood pressure monitors. This application
note demonstrates how to use the PSoC to build an
oscillometric blood pressure monitor.
Table 1. Non Invasive Methods to Monitor Blood Pressure
Method
Palpatory
(Riva-Rocci)
Palpable pulse when cuff pressure equals
systolic pressure (SP)
Auscultatory
Based on sound waves generated from artery
Ultrasonic
Based on frequency difference between
transmitted and reflected ultrasound wave
when passed through arteries
Tonometry
When the blood vessel is partly collapsed, the
surrounding pressure equals the artery
pressure. Measured using an array of pressure
sensors and the cuff is around the wrist
Oscillometric
(Popular
and widely
used)
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Non-invasive Principle
Document No. 001-58128 Rev. *C
The intra-arterial pulsation is transmitted via
cuff to transducer (example, piezo-electric). SP
and DP are estimated from the amplitudes of
the oscillation by using an empirical algorithm.
Oscillometric method is used in almost all
portable blood pressure monitors
1
Blood Pressure Monitor with PSoC®
Figure 1. Oscillometric Pulses Vs Cuff Pressure
Operation Principle
The blood pressure monitor operates on the following
principles.

The cuff is worn around the upper arm and it is inflated
beyond the typical systolic pressure.

It is then deflated. The pressure starts decreasing,
resulting in blood flow through the artery; this makes the
artery to pulsate.

The pressure measured on the device during onset of
pulsations defines the systolic blood pressure.

Then the cuff pressure is reduced further. The
oscillations become increasingly significant, until they
reach maximum amplitude.

The pressure at the maximum amplitude of these
oscillations defines the average blood pressure.
Block Diagram

The oscillations start decreasing as the cuff pressure
reduces. The pressure at this point defines the minimal
blood pressure or diastolic blood pressure.
Figure 2 shows the block diagram of the blood pressure
monitor using PSoC. The device uses oscillometric method
to determine systolic and diastolic pressures. This system
includes the following blocks:
This method of measuring blood pressure is the oscillometric
method. It is often used in automatic blood pressure monitor
devices because of its excellent reliability. Figure 1 shows
the variation of the artery oscillations as the cuff pressure is
reduced; it also shows the systolic and diastolic points.
Estimation of systolic and diastolic pressure is done using
various empirical algorithms. This application note
demonstrates PSoC hardware’s capability to perform blood
pressure monitoring by extracting the oscillometric waveform
and doesn’t describe the algorithm used to extract blood
pressure from the oscillometric waveform. This is primarily
because all reliable algorithms are complex and deserve a
separate discussion and empirical algorithms don’t give a
reliable value across a good number of people.
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





Pressure sensor
Amplifier
Filter
Multiplexer and ADC
Pneumatics
Display
Document No. 001-58128 Rev. *C
2
Blood Pressure Monitor with PSoC®
Figure 2. Block Diagram
Figure 3.Transfer Characteristics of Sensor
Implementation of these blocks in PSoC is described in the
following sections.
Pressure Sensor
The pressure sensor for blood pressure monitoring system
should have the following characteristics:

Measure pressures from 0 mmHG (0 Kpa) to 300 mmHg
(40 Kpa).

Gauge type, because blood pressure in relation to
atmospheric pressure
MPX2053 (piezoresistive pressure sensor from free scale) is
used in this example. It gives differential output with
maximum measurable pressure range of 50 Kpa. It has a
transfer characteristic of (20 mV/50 Kpa) 0.4 mv for every
1 Kpa change in pressure or 53 µV per mmHG with Vs=5 V
as illustrated in Figure 3.
Amplifier
The sensor output is in the order of a few micro volts. Three
opamp topology instrumentation amplifiers are used to
amplify the pressure signal. It provides a gain of 93.
Gain = Diff Gain * Conversion Gain = 48 * 1.98 = 93
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Document No. 001-58128 Rev. *C
3
Blood Pressure Monitor with PSoC®
Filter
series are replaced by one equivalent inverting amplifier with
gain equal to k.
The sensor output consists of two signals: cuff pressure
signal and oscillometric signal. The oscillometric signal has
frequency components between 0.3 Hz to 20 Hz. Two stage
filters are used to filter out the oscillometric pulses.
Using Kirchhoff's laws and simple algebra, the frequencyresponse function for the circuit shown in
Figure 5 can be written as follows:
First Stage
A high gain AC filter is implemented in the first stage.
Topology of this stage is illustrated in Figure 4.
a2 p 2  a1 p
K ( p)  
.k
b2 p 2  b1 p  b0
This circuit uses only two analog I/O. An Inverting Amplifier
(AMPINV) User Module is used to achieve inverted output as
opposed to input. This allows the connection of output to
input via the additional RC-circuit to form negative feedbackcoupling. Capacitor C2 suppresses the AC component of the
feedback signal. The DC component passes from output to
input without any changes, which forms 100% DC negative
feedback. This permits the output DC voltage to be close to
analog ground, independent of the amplifier’s input offset
voltage.
a2  C1C2 ( R2 R3  R2 R4  R3 R4 )
Figure 4. High AC Gain Amplifier
a1  C1 ( R2  R3)
Equation 1
b2  C1C2 R3 ( R1  R2 )  C1C2 R4 ( R1  R2  R3  kR1 )
b1  C1 ( R1  R2  R3 )  C2 R3  kC1R1  (1  k )C2 R4
b0  k  1
Equation (1) describes a combination of high-pass and
band-pass filters with the same roll-off frequency.
The filter’s cutoff is set around 1 Hz. This filter removes all
DC components and gives the AC signal a sufficient gain.
The output of the first stage has unwanted high frequency
components.
r 2 
b0
b2
 Equation 2
The gain at higher frequencies is equal to
The circuit in Figure 4 seems to be very simple.
Nevertheless, it is not easy to estimate the gain-frequency
characteristic of the whole amplifier due to frequencydependent feedback. For analysis purposes, the amplifier
circuit should be slightly modified as shown in
k
a2
b2
. The
frequency response is shown in Figure 6.
Figure 6. Frequency Response
Figure 5.
Figure 5. Amplifier Equivalent
The resistor R1 is added, taking into account the AC signal
source output resistance.PGA and AMPINV coupled in
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Second Stage
High frequency components are removed using two pole low
pass filters implemented inside PSoC. This filter is
constructed using two switched capacitor blocks. The filter’s
cutoff is set at 50 Hz with a 0 dB gain.
Document No. 001-58128 Rev. *C
4
Blood Pressure Monitor with PSoC®
Multiplexer and ADC
Pneumatics
DC Pressure signal and the oscillometric are multiplexed to
ADC inside the PSoC. The MUX selects one of these signals
to a 13-bit incremental ADC, which runs at a sampling rate of
30 samples/second. Correlated double sampling, described
in the Cypress application note AN2226 is implemented to
avoid offset errors. A low-pass IIR filter is implemented in
software. This averages and effectively reduces the noise
from the input signal. For details on modifying the filter
constant and other IIR techniques, see application note
AN2099 Single-Pole IIR Filters. To Infinity and Beyond!
Pneumatics forms the main part of any blood pressure
monitoring system. Pneumatics of a typical monitor has the
following:




Cuff
Air chamber
Rolling pump
Solenoid valve
The cuff is worn around the upper arm; it detects the change
in pressure due to pulsation of artery. Cuff is connected to
pressure sensor through air chamber, which in turn connects
to the solenoid valve and rolling pump. Rolling pump inflates
the cuff. Solenoid valve deflates the cuff at a defined rate.
Usually the deflation rate is lowered if more samples of
oscillometric pulses are needed and vice versa. Figure 7
shows the pneumatics setup used to build a typical blood
pressure monitor.
Display
Using serial UART communication, the oscillometric pulses
are recorded with reference to pressure in cuff. The UART
transmitter runs at a baud rate of 19200. Oscillometric
pulses and pressure in cuff are recorded during deflation.
Figure 8 shows the Oscillometric Pulses and Pressure
Signal.
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Document No. 001-58128 Rev. *C
5
Blood Pressure Monitor with PSoC®
Figure 7. Blood Pressure Monitor with PSoC
Note Medical demo daughter card is built for internal cypress demo and the same is not sold in cypress web.
www.cypress.com
Document No. 001-58128 Rev. *C
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Blood Pressure Monitor with PSoC®
Figure 8. Oscillometric Pulses and Pressure Signals
Figure 9. Device Schematic
Error! Reference source not found. shows the
schematics of blood pressure monitor daughter card built
www.cypress.com
to
interface
CY8CKIT-001
-
Document No. 001-58128 Rev. *C
with
Port
Development
Kit
B
(DVK)
of
with
7
Blood Pressure Monitor with PSoC®
CY8CKIT-008. P0x, P2x refers to the ports of PSoC1 in
the DVK. The rolling pump and valve used are from koge
and the part numbers are KPM12A and KSV04A-3C.
Table 2 provides details of each net.
Table 2. Net Details
format so that you can directly import it to MS excel in
*.csv format to plot graphs and find systolic and diastolic
pressures. A reference MS Excel sheet is attached this
project. The pressure in the cuff is calculated as follows.
Transducer Voltage =
Net name
Details
Slope =
P07_PGA_INPUT
Input of high AC filter
P01_ SENSOR-
Negative input from pressure sensor
P03_I
Amplified pressure sensor output
which acts as input to filter
P06_SENSOR+
Positive input from pressure sensor
P20_PUMP DRIVE
PWM output to drive pump
P21_VALVE DRIVE
PWM output to drive valve
P17_TX
UART transmitter
Amplified _ Transducer _ Voltage
Total _ DC _ Gain
20mV
 0.4 mV/Kpa
50 Kpa
This is the transfer characteristic of sensor.
Pr essure( Kpa) 
Transducer _ Voltage
slope
Pressure(mmHg) = Pressure(Kpa) 
760mmHg
101.325 Kpa
Pressure(mmHg) = Amplified _ Transducer _ Voltage * 201.62
Working
The PWMs are set to 100% duty cycle for inflating the cuff.
When the pressure inside the cuff exceeds a threshold,
say 180 mmHg, the cuff is deflated. The systolic and
diastolic pressures are determined based on the filtered
(ADC) output.
Project Description
A project is attached with this application note. This project
helps to extract the pressure and oscillometric waveform
from which you can find the BP by manual inspection or
using your own algorithm. The project inflates the cuff to
180 mmHG. Then deflates the cuff at a constant rate and
simultaneously puts the pressure values and filtered
output in counts through UART in a comma separated
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Calibration
The captured ADC counts of pressure signal can be
calibrated for the range 0-300 mmHG pressure range
using a standard pressure meter. The process involves
plotting the ADC counts against pressure measured in
standard pressure meter in steps of 2-5 mmHG. The data
can be used to derive trend line equation in form of
polynomial with required order or the data can be used as
a lookup table. This is will compensate errors due to gain
and offset of the whole signal chain. The attached XL
sheet contains the calibration data for the setup in
Figure 7.
Document No. 001-58128 Rev. *C
8
Blood Pressure Monitor with PSoC®
Figure 10. PSoC Internal Routing and Placement
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Document No. 001-58128 Rev. *C
9
Blood Pressure Monitor with PSoC®
About the Author
Summary
The blood pressure monitor described in this application
note is constructed using PSoC 1 with no active external
components. Devices used for medical diagnostic
applications must undergo a rigorous documentation and
qualification program to meet applicable interruption
medical safety efficacy standards.
Related Application Notes
Name:
Sanjeev Kumar K.
Title:
Applications Engineer
Background:
Sanjeev has a Bachelor’s degree in
Electronics and Communication from
the College of Engineering, Guindy,
and Chennai. He currently works on
PSoC based applications at Cypress.
Contact:
[email protected]
®
AN2226 - PSoC 1 - Using Correlated Double Sampling to
Reduce Offset, Drift, and Low Frequency Noise
®
AN2099 - PSoC 1, PSoC 3, and PSoC 5LP - Single-Pole
Infinite Impulse Response (IIR) Filters
www.cypress.com
Document No. 001-58128 Rev. *C
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Blood Pressure Monitor with PSoC®
Appendix A
Segment LCD Glass Drive and Capacitive Sensing
Portable blood pressure monitors have custom LCD glasses and a few buttons for user input. Apart from monitoring blood
pressure, some parts in the PSoC family can drive LCD glasses directly without any driver and also do capacitive touch
sensing. Using these parts enable a greater reduction in total bill of materials. For details about LCD drive and capacitive
touch sensing refer application note AN56384.
www.cypress.com
Document No. 001-58128 Rev. *C
11
Blood Pressure Monitor with PSoC®
Document History
®
Document Title: Blood Pressure Monitor with PSoC - AN58128
Document Number: 001-58128
Revision
ECN
Orig. of
Change
Submission
Date
Description of Change
**
2824451
KUK
1/18/10
New application note
*A
3126383
KUK
01/03/2011
Comment added to Figure 5.
Project updated to Latest PSoC designer 5.1 and author profile updated.
*B
3940502
KUK
03/27/2013
Updated Software Version as “PSoC® Designer™ 5.3”.
Updated Operation Principle.
Updated Block Diagram (Updated Filter, removed “Safety Timer” and
“Pneumatics”).
Added Working.
Renamed “Software” as Project Description and updated the same section.
Added Calibration.
Updated Summary.
Updated Related Application Notes (Removed references of AN2320 as it is an
obsolete document).
Updated in new template.
Algorithm to determine systolic and diastolic pressures have been removed.
*C
4744464
www.cypress.com
SSHH
04/27/2015
This document is obsolete.
Document No. 001-58128 Rev. *C
12
Blood Pressure Monitor with PSoC®
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Document No. 001-58128 Rev. *C
13