Blood Pressure Controller

7/28/2019 Blood Pressure Controller

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Introduction|High Level Design|Hardware Design|Software Design|Results|Conclusions|PhotosAppendix A: Code |Appendix B: Schematics |Appendix C: Parts Listing |Appenddix D: Tasks|References

troduction

Our final project is to design and build a portable blood pressure monitor device that can measure a user's blooessures and heart rate through an inflatable hand cuff. The device is consisted of three main parts: external hardwaruch as cuff, motor, valve, and lcd), analog circuit, and microcontroller. The anolog circuit converts the pressure valside the cuff into readable and usable analog waveforms. The MCU samples the waveforms and performs A/D

onvertion so that further calculations can be made. In addition, the MCU also controls the operation of the devices sthe button and lcd display. Since we have the word 'portable' in our title, for sure all of the components are put togone package which allows a user to take it anywhere and perform a measurement whenever and wherever he/she

ants.

It is undeniable that nowadays people are more aware of the health conditions. One of the most widely used

ethods to test the health conditions of an individual is to measure his/her blood pressures and heart rate. We, as oneose who are concerned about their health, decided to work on this subject matter because we would like to buildmething that is useful and useable in real life.

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igh Level Design
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//people.ece.cornell.edu/land/courses/ece4760/FinalProjects/s2005/ww56_ws62/Final%20Project%20Web/index.html#Photoshttp://people.ece.cornell.edu/land/courses/ece4760/FinalProjects/s2005/ww56_ws62/Final%20Project%20Web/index.html#Conclusionshttp://people.ece.cornell.edu/land/courses/ece4760/FinalProjects/s2005/ww56_ws62/Final%20Project%20Web/index.html#Resultshttp://people.ece.cornell.edu/land/courses/ece4760/FinalProjects/s2005/ww56_ws62/Final%20Project%20Web/index.html#Softwarehttp://people.ece.cornell.edu/land/courses/ece4760/FinalProjects/s2005/ww56_ws62/Final%20Project%20Web/index.html#Hardwarehttp://people.ece.cornell.edu/land/courses/ece4760/FinalProjects/s2005/ww56_ws62/Final%20Project%20Web/index.html#Highhttp://people.ece.cornell.edu/land/courses/ece4760/FinalProjects/s2005/ww56_ws62/Final%20Project%20Web/index.html#Introduction


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How blood pressures are measured

Usually when the doctor measures the patient's blood pressure, he will pump the air into the cuff and use theethoscope to listen to the sounds of the blood in the artery of the patient's arm. At the start, the air is pumped to bebove the systolic value. At this point, the doctor will hear nothing through the stethoscope. After the pressure is rele

adually, at some point, the doctor will begin to hear the sound of the heart beats. At this point, the pressure in the cuorresponds to the systolic pressure. After the pressure decreases further, the doctor will continue hearing the sound (fferent characteristics). And at some point, the sounds will begin to disappear. At this point, the pressure in the cuff

orresponds to the diastolic pressure.

To perform a measurement, we use a method calledoscillometric. The air will be pumped into the cuff to

ound 20 mmHg above average systolic pressure (about 120 mmHg for an average). After that the air will be slowlyleased from the cuff causing the pressure in the cuff to decrease. As the cuff is slowly deflated, we will be measurine tiny oscillation in the air pressure of the arm cuff. The systolic pressure will be the pressure at which the pulsationarts to occur. We will use the MCU to detect the point at which this oscillation happens and then record the pressure cuff. Then the pressure in the cuff will decrease further. The diastolic pressure will be taken at the point in which

cillation starts to disappear.

Hardware diagram

The diagram above shows how our device is operated. The user will use buttons to control the operations of thhole system. The MCU is the main component that controls all the operations such as motor and valve control, A/D

onversion, and calculation, until the measurement is completed. The results then are output through and LCD screen

e user to see.

Analog Circuit

The analog circuit is used to amplify both the DC and AC components of the output signal of pressure transdu that we can use the MCU to process the signal and obtain useful information about the health of the user. The pres

ansducer produces the output voltage proportional to the applied differential input pressure. The output voltage of thessure transducer ranges from 0 to 40 mV. But for our application, we want to pump the arm cuff to only 160 mmH


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pproximately 21.33 kPa). This corresponds to the output voltage of approximately 18 mV. Thus, we choose to ampe voltage so that the DC output voltage of DC amplifier has an output range from 0 to 4V. Thus, we need a gain of

pproximately 200. Then the signal from the DC amplifier will be passed on to the band-pass filter. The DC amplifiemplifies both DC and AC component of the signal (it's just a regular amplifier). The filter is designed to have large g

around 1-4 Hz and to attenuate any signal that is out of the pass band. The AC component from the band-pass filte

e most important factor to determine when to capture the systolic/diastolic pressures and when to determine the heate of the user. The final stage is the AC coupling stage. We use two identical resistors to provide a DC bias level atpproximately 2.5 volts. The 47 uF capacitor is used to coupling only AC component of the signal so that we can proe DC bias level independently.

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ardware Design

Pressure Transducer

We use the MPX2050 pressure transducer from Motorola to sense the pressure from the arm cuff. The pressuransducer produces the output voltage proportional to the applied differential input pressure. We connect the tube froe cuff to one of the inputs and we leave another input open. By this way, the output voltage will be proportional to fference between the pressure in the cuff and the air pressure in the room. The transfer characteristic is shown in fig

Figure 1: Output voltage vs. Differential input pressure

DC Amplifier

Since the output voltage of the pressure transducer is very small, we have to amplify the signal for furtherocessing. We use the instrumentation amplifier AD620 from Analog Devices. The resistor R G is used to determin

ain of the amplifier according to the equation . Since we need the gain of approximately 200, we chooe resistor R G to be 240 ohms. This will give us the gain of 206 according to the equation. However, we have meas
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e gain from the finished circuit, and the measured gain is 213. The schematic of the amplifier is shown in figure 2.

Figure 2: Schematic of DC amplifier

Band-pass Filter

The band-pass filter stage is designed as a cascade of the two active band-pass filters. The reason for using twages is that the overall band-pass stage would provide a large gain and the frequency response of the filter will have

harper cut off than using only single stage. This method will improve the signal to noise ratio of the output. Thehematics for both filters are shown in figure 3.

Figure 3: Bandpass Filter Stage

rst Band-pass filter :

The lower frequency cutoff is


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The higher frequency cutoff is

The mid-band gain of the first filter is

econd Band-pass filter:

The lower frequency cutoff is

The higher frequency cutoff is

The mid-band gain of the first filter is

Thus for the band-pass filter stage, the overall gain is 399.6. Combining this gain with the gain from the DC

mplifier, the total AC gain for the circuit is .The choice of high and low cut-off frequency is good enoughve us very clean AC waveform.

AC coupling stage

The ac coupling stage is used to provide the DC bias level. We want the DC level of the waveform to locate atpproximately half Vdd, which is 2.5 V. The schematic for AC coupling stage is shown in figure 4. Given this bias leis easier for us to process the AC signal using the on-chip ADC in the microcontroller.

Figure 4: AC coupling stage for DC bias level


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The AC output from this stage will be passed on to the analog-to-digital converter in the Mega32 microcontrohe image from the laboratory bench is shown in figure 5. We can see that it is very nice and clean.

Figure 5: AC Waveform

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oftware Design

Design for the operating control

he block diagram for the operating control is consisted of a total of 7 states. We first start at the START state whereogram waits for the user to push the white button of the device. Once the white button has been pushed, theeasurement process begins by inflating the hand cuff. While the cuff is being inflated, if the user feels very

ncomfortable or painful, he/she can push the grey button(emergency button) to stop the motor, quickly deflate the cund stop the measurement. This will ensure that the safety of the user is well maintained while using the device. Anyhthe cuff-inflating procedure goes smoothly, the air will be pumped into the cuff until the pressure inside the cuffaches 160 mmHg. After that, the motor will be stopped and the air will be slowly released from the cuff. Again, at t

oint, the user can abort the process by pressing grey button. Once the MCU has obtained the values of systolic, diasnd heart rate, the valve will be open to release air from the cuff quickly. Then it will report the result of the measurey displaying the obtained data on the LCD screen. After that if the black button is pushed the program will return toTART state again waiting for the next measurement. Note that if the emergency button is pushed, the black button n

be pushed in order to return to the start state.


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Design for measuring the metrics

Once the motor pumps the air into the cuff until the pressure exceeds 160 mmHg, the motor then stops pumpinore air and the cuff is deflated through the slightly-opened valve. The pressure in the cuff starts decreasing

pproximately linearly in time. At this point, the program enters the measurement mode. The MCU will looks at the A

gnal through the ADC0 pin and determines the systolic, diastolic pressure values and the heart rate of the userspectively. For this project, we perform the measurement using the oscillometric method, in which the programonitors the tiny pulsations of the pressure in the cuff. The state diagram of the measurement is shown in figure 7


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Figure 6: State Diagram for Measurement

1) Systolic Pressure Measurement

After the motor pumps the pressure up to 160 mmHg which is approximately more than the systolic pressure oormal healthy people, the cuff starts deflating and the program enters Sys_Measure state. In this state, the program woks at the AC waveform from ADC0 pin. When the pressure in the cuff decreases to a certain value, the blood begi

ow through the arm. At this time if we look at the oscilloscope, we will see the onset of the oscillation. The systolicessure can be obtained at this point.

The way we program this is that we set a threshold voltage of 4V for the AC waveform. At the start, there is nulse and the voltage at the ADC0 pin is constant at approximately 2.5 V. Then when the pressure in the cuff decreasntil it reaches the systolic pressure value, the oscillation starts and grows. We then count the number of pulses that haximum values above the threshold voltage. If the program counts up to 4, the program enters the Sys_cal state. At ate, the program records the DC voltage from pin ADC1. Then it converts this DC voltage value to the pressure in tuff to determine the systolic pressure of the patient.

From the transfer characteristic of the pressure transducer and the measured gain of the DC amplifier, we can

etermine the systolic pressure by looking at the DC voltage of the ADC1 pin. We will explain the conversion proced

ere. Let's the DC voltage that we read off of the ADC1 pin beDC_voltage', and the gain of the DC amplifier be

DC_gain'. Then the differential voltage that comes out of the DC amplifier is calculated as

. From the pressure transducer's transfer characteristic given in figure 1 in thercuit design part, we can calculate the pressure based on the transducer_voltage. The slope of the typical curve is


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lculated as . Thus, the pressure in the cuff in the unit of kPa can be calculated as

. Then we can convert the pressure back to mmHg unit by multiplying by

.Thus the pressure in the mmHg unit is expressed as ombining these conversions all together, we obtain the formula for converting the DC voltage to the pressure in the

.

After the program finishes this calculation, it enters the Rate_measure state to determine the pulse rate of theatient.

2) Pulse Rate Measurement

After the program finished calculating the systolic pressure, then it starts monitoring the pulse rate of the patieWe choose to determine the pulse rate right after determining the systolic pressure because at this point the oscillation

e waveform is strongest. The program samples the AC waveform every 40 millisecond. It then records the time intehen the values of the AC waveform cross the voltage value of 2.5 volts. The program then takes the average of fivetervals so that the heart rate will be as accurate as possible. The variable used for counting the number of time intercount_average as shown in the state diagram. After the heart rate is determined, the program then enters theias_measure state, in which it tries to measure the diastolic pressure of the patient.

3) Diastolic Pressure Measurement

After the pulse rate is determined, the program enters the Dias_Measure state. In this state, the program is stillmpling the signal at every 40 millisecond. We then define the threshold for the diastolic pressure. While the cuff iseflating, at some point before the pressure reaches diastolic pressure, the amplitude of the oscillation will decrease. etermine the diastolic pressure, we record the DC value at the point when the amplitude of the oscillation decreases elow the threshold voltage. This is done by looking at the time interval of 2 seconds. If the AC waveform does not gbove the threshold in 2 seconds, it means the amplitude of the oscillation is actually below the threshold. The DC van then be converted back to the pressure in the arm cuff using the same procedure as described in the Systolic Preseasurement section.

Please note that determining the diastolic pressure is quite difficult and ambiguous since the voltage thresholdaries from person to person. Thus, we have to adjust the voltage threshold that we use so that the value of diastolicessure that we obtain corresponds to the known value we usually get when we measure it using the available

ommercial product.

After the program finishes calculating the diastolic pressure, it will display the information acquired from theeasurement on the LCD. Then the program will open up the valve and the cuff will deflate quickly. The measureme

ow finished.


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esults

The results of the project are as expected and satisfactory to us. If the user stays still during the operation, the

evice can measure blood pressures(both systolic and diastolic) and heart rate without any problem.

Duration of measurement

From the start until all the measurements are done, it takes about 1.5 minutes. However, this also depends on edividual and how the cuff is worn. For each person, the amplitude of the waveforms may differ causing the operatinme to vary. Still, the difference is small and is usually within 10 seconds.

Accuracy

As mentioned earlier, all the measurements are mainly dependent on the waveforms from the circuit and the

essure sensor is very sensitive to even a slight movement of the user. As a result, it is possible that sometimes theevice fails to obtain the desired data, especially if the user does not stay still or wear the cuff improperly.

Regarding the three result values (systolic, diastolic and heart rate), some of them has more success rate than thers. For the heart rate, the success rate is very high for getting an accurate value. To find the heart rate, we need toe period of the AC waveform. And since the period of heart rate stays pretty much constant through out theeasurement, it is relatively easy to obtain an accurate result. To find the pressure values, however, are harder becauey depend on the amplitude of the waveform, and the amplitude varies a lot during the measurement. However, if ther stays still and wears the cuff right, the measurement are usually successful 8 time out of 10 tries (80% success ra

Another topic that is worth mentioning is that the method of measurement that we used is called the oscillomet

ethod. It is usually deployed in commercial products due to the reliability. However, this method is not as accurate e auscultatory method, in which the doctor uses the mocrophone to listen to the noise in the artery.

Safty in Design

Since this is a medical instrumentation device, the safety of the user is the first concern to us. The cuff whileiven by a 5 volts motor can squeeze the arm really hard and cause injury if being used improperly. So in our device

ave 3 levels of security, making sure that the operation can be aborted by the user at anytime.

For the first safety design, the microcontroller is programmed in the way such that if the pressure in the cuff iseater than 160 mmHg, the motor will stop. For most people, the pressure at 160 mmHg will only cause a little

scomfort to the arm. This desgin makes sure that the pressure inside the cuff will never exceed the maximum limit 60 mmHg.

The second safety design is to provide an emergency button for the user. While the motor is pumping and the being inflated, if the user encounters too much discomfort or pain, he/she can press this button to stop the operation

mmediately. The motor will be stopped and the valve will be opened to release the air out of the cuff.

However, we still think that only a pushbutton is not enough for the safety of the user. This is because the


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mergency button still relies on the operating system of the program in the MCU. Even though we have no bug in ouogram, if something goes wrong with the MCU or the button connections, there is a possibility that the emergency

utton becomes unusable. So we agree that we must have another swtich that control the device physically. This switust be able to disconnect the circuit from the power supply immediately. This way it is certain that the user will be stop the operation even when the emergency button does not work. And this switch is the on-off power switch that

ave on the device.

Other than the cuff and motor concerns, our project is very safte to use because it is very well packaged in a plnclosure. The device is run by low-voltage(9 volts) battery which cannot cause any major harm to human body.

Interference with other people's designs

Since our project only performs measurements on an individual (user), there should not be any interferenceoduced by the device besides the sound of the motor running (which is very quiet compared to regular motors).

Usability

Our project should be useable to most adults, since it is basically a regular blood pressure moniotor sold in thearkets nowadays. The instructions LCD screen are pretty straight forward and easy to understand. Since this device

uilt to be portable, it can be used anywhere and any time as long as the battery still has power.

The cuff that we use is appropriate for the average adult arm size (9-13 inches in circumference). Thus the armat is out of this range may not give the accurate measurement. Due to our budget constraints, we do not have many ze for our project

For people with health problem, especially on blood pressure or heart, it is not recommended to use this deviceecause we have only tested it with healthy people. Therefore, for the maximum safety of the user, this device can on

e used by the people who do not have medical problems on heart and circulartory system.

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onclusions

Analysis and expectations

Most of the designs that we proposed before we started the project are met in our final result. In fact, we are reappy and satisfied with the final result of our project. The measurements are acceptably accurate (please see 'Accuraction in results part). The operations of the device are reliable and have not produced any major problems. The pow

onsumption of the device is decent as we have already tried lots of measurements(more than 20) and the set of two 9olt batteries has not died yet.

Regarding the batteries and power consumption, we encountered a big problem in this issue while testing theevice. At the first place, we used one battery to power the MCU board, and the other to power the circuit, valve andotor. However after a couple of tries, the battery that powered the circuit and motor lost its power and could not proconstant voltage during the measurement. In other words, the voltage across that battery drops constantly as the mons. As a result, the voltage that feeds all the chips in the circuit are not high enough (the AD620 and OPA2277 requ


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V and -5V) and causes the circuit to mulfunction.

So we fixed this problem by separating the battery that runs the valve and motor from the rest of the circuit. The use the battery that supplies the MCU board to power the circuit instead. Now that the battery that supplies the va

nd the motor is separated, it ensures that there will be no voltage drop in the circuit while the motor is running. This

ay, the power consumption of the 2 batteries will be more balanced, since the circuit and the MCU do not consumeuch power.

There is only one proposed feature that is missing from our project. It is the option that let the user to output thulse waveform(AC component) on the TV. If we want to output the signal on TV, we will need another Mega32 andc board. However, due to the limited budget of $50, this could not be done.

Intellectual property considerations

All the circuits and the codes are originally designed by us. The topologies of the circuits are the one discussedandard textbooks. We do not use other people's codes or circuit topologies from other people designs. Thus, we are

at we do not violate any intellectual property of the existing design.

Ethical considerations

We have tried to ensure that our design conforms to the IEEE codes of Ethics. Here are the considerations that we takmind.

1)to accept responsibility in making engineering decisions consistent with the safety, health and welfare ofthe public, and to disclose promptly factors that might endanger the public or the environment

-We have been aware of the danger of our device in which the arm cuff may injure the patients.

Thus, it is advised that the patients use this product under the monitor of qualified physicians.And during the design, we are concerned with our safety. With regard to this issue, we make sure thatwe can abort the operation at any time by pushing the button or the disconnecting the power supply.

2) to avoid real or perceived conflicts of interest whenever possible, and to disclose them toaffected parties when they do exist

-There is no conflict with interest between any of the design team during this project.

3) to be honest and realistic in stating claims or estimates based on available data

-We base all of our design on available data supplied by the vendors and the measurement in the labwithout faltering any bit of information.

4) to reject bribery in all its forms

-Of course, there is no bribery issue for our project.


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5) to improve the understanding of technology, its appropriate application, and potential consequences

-Through this project, we have improved our understanding of how to benefit the society through our engineeringexpertise. Furthermore, we improve a lot of our knowledge in analog and digital design in the practical world.

6) to seek, accept, and offer honest criticism of technical work, to acknowledge and correct errors, and tocredit properly the contributions of others

We have been opened to any criticisms and help from the instructor, TA and colleagues through the designphase and we very appreciate their kindly help.

7) to avoid injuring others, their property, reputation, or employment by false or malicious action;

We have always been trying to avoid injuring others by performing the experiment on ourselves.And of course, our safety is an important concern as we mentioned earlier in 1).

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hotos

The hard work before the product is done


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ppendix A: Code

Because our code is quite long, please clickhereto download the file

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ppendix B: Schematics

Figure A1: Schematics

of Analog Circuit

All the OPA277 is powered by 5V/-5V supply
http://people.ece.cornell.edu/land/courses/ece4760/FinalProjects/s2005/ww56_ws62/Final%20Project%20Web/index.html#tophttp://people.ece.cornell.edu/land/courses/ece4760/FinalProjects/s2005/ww56_ws62/Final%20Project%20Web/index.html#tophttp://people.ece.cornell.edu/land/courses/ece4760/FinalProjects/s2005/ww56_ws62/Final%20Project%20Web/code.chttp://people.ece.cornell.edu/land/courses/ece4760/FinalProjects/s2005/ww56_ws62/Final%20Project%20Web/code.chttp://people.ece.cornell.edu/land/courses/ece4760/FinalProjects/s2005/ww56_ws62/Final%20Project%20Web/code.chttp://people.ece.cornell.edu/land/courses/ece4760/FinalProjects/s2005/ww56_ws62/Final%20Project%20Web/index.html#tophttp://people.ece.cornell.edu/land/courses/ece4760/FinalProjects/s2005/ww56_ws62/Final%20Project%20Web/index.html#tophttp://people.ece.cornell.edu/land/courses/ece4760/FinalProjects/s2005/ww56_ws62/Final%20Project%20Web/index.html#tophttp://people.ece.cornell.edu/land/courses/ece4760/FinalProjects/s2005/ww56_ws62/Final%20Project%20Web/code.chttp://people.ece.cornell.edu/land/courses/ece4760/FinalProjects/s2005/ww56_ws62/Final%20Project%20Web/index.html#top


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Figure A2: Schematic of Motor and Valve Control


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Figure A3: Schematic of Supply Voltage Circuit

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ppendix C: Parts Listing

Components Quantity Price

Mega32 Microcontroller 1 $8.00Custom PC board 1 $5.00Hand-cuff 1 $16.0

Solder board 1 $2.50White button 1 $1.31Grey button 1 $1.31Black button 1 $1.31On-Off switch 1 $1.67LCD 1 $8.00Pressure TransducerMPX2050

1 Free, Sample


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Instrumentation AmplifierAD620

1 Free, Sample

Op-ampOPA2277

1 Free, Sample

9V to -5V Voltage regulator 2 Free, Sample

9V to 5V Voltage regulator 1 Free, From labResistors and Capacitors Free, From labLED 1 Free, From labMotor and Valve 2 Free, Used

Total $45.10

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ppendix D: Tasks

For this project, both of us work very closely with each other and spend more than 50 hours in the lab. For the

esign of analog circuit, we worked on the design together during the first two weeks of the final project. Then we spp the tasks of writing the code. Warut is responsible for writing the motor and valve control codes. Woradorn issponsible for the codes for the measurement of the metrics (systolic, diastolic and heart rate).

After we finished the design, we split up the task for soldering the components into the proto-board. Warut issponsible for soldering the proto-board for the microcontroller. Woradorn is responsible for the soldering of the anarcuit board. Furthermore, we both contribute equally to the soldering of the supply circuit (all the voltage regulatorsattery). For the package design, Warut takes care all of that.

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eferences

Data Sheet

D620 (for DC Amplifier) from Analog Devicestp://www.analog.com/UploadedFiles/Data_Sheets/37793330023930AD620_e.pdf

PX2050 (Pressure Transducer) from Motorolatp://www.elemar.pl/mpx2052.pdf

PA2277 (used in band-pass filter design) from Texas Instrument

tp://focus.ti.com/lit/ds/symlink/opa277.pdf

T178NR105V (voltage regulators) from Texas Instrumenttp://focus.ti.com/lit/ds/symlink/pt78nr105.pdf

Reference Site

ow to measure the blood pressure
http://people.ece.cornell.edu/land/courses/ece4760/FinalProjects/s2005/ww56_ws62/Final%20Project%20Web/index.html#tophttp://people.ece.cornell.edu/land/courses/ece4760/FinalProjects/s2005/ww56_ws62/Final%20Project%20Web/index.html#tophttp://people.ece.cornell.edu/land/courses/ece4760/FinalProjects/s2005/ww56_ws62/Final%20Project%20Web/index.html#tophttp://people.ece.cornell.edu/land/courses/ece4760/FinalProjects/s2005/ww56_ws62/Final%20Project%20Web/index.html#tophttp://www.analog.com/UploadedFiles/Data_Sheets/37793330023930AD620_e.pdfhttp://www.elemar.pl/mpx2052.pdfhttp://focus.ti.com/lit/ds/symlink/opa277.pdfhttp://focus.ti.com/lit/ds/symlink/pt78nr105.pdfhttp://focus.ti.com/lit/ds/symlink/pt78nr105.pdfhttp://focus.ti.com/lit/ds/symlink/opa277.pdfhttp://www.elemar.pl/mpx2052.pdfhttp://www.analog.com/UploadedFiles/Data_Sheets/37793330023930AD620_e.pdfhttp://people.ece.cornell.edu/land/courses/ece4760/FinalProjects/s2005/ww56_ws62/Final%20Project%20Web/index.html#tophttp://people.ece.cornell.edu/land/courses/ece4760/FinalProjects/s2005/ww56_ws62/Final%20Project%20Web/index.html#top


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tp://www.drbloodpressure.com/05-mesurer8.shtml
http://www.drbloodpressure.com/05-mesurer8.shtmlhttp://www.drbloodpressure.com/05-mesurer8.shtml

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Blood Pressure Controller

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