ece525.pdf

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Udayan V. Bapat ECE 525 – Medical Instrumentation

i

Project 1

ECG Amplifier

By

Udayan V. Bapat

M.S. Computer Engineering

North Carolina State University

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ii

Contents

1 Introduction……………………………………………………………………… . 1

2 Project Specifications ………………………………………………………….... 2

3 System Design Considerations………………………………………................... 3

3.1 Sources of Interference………………………………………………………… 3

3.2 Interference Elimination Strategies……………………………………………. 3

3.3 Circuit Design………………………………………………………………… . 4

3.4 Circuit Schematic……………………………………………………………… 5

4 Testing and Results……………………………………………………………… . 8

4.1 Simulation Using B2spice…………………………………………………… .. 8

4.2 Debugging using Signal Generator …………………………………………… 8

4.3 ECG Amplifier Output ……………………………………………………… .. 9

5 Design issues/ Implementations / Practical Considerations…………………… 10

6 Component Listing……………………………………………………………… . 12

7 Referances………………………………………………………………………… 13

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1

Chapter 1

Introduction

The aim of this project is to record electrocardiogram. The electrocardiogram is the potential developed by

heart and measured on the body-surface. Several "waves" occur with each heartbeat. The waves are labele

QRS, and T and correspond to electrical activity when the heart's atria contract (P); the ventricles cont

(QRS) and when the ventricles repolarize (T). The QRS complex duration is about 80 milliseconds. Typicalhas amplitude of about 1 mVp-p. The P-wave has amplitude of about 100 microvolt. The wave-complex rep

with each heartbeat approximately 72 times/minute.

The typical ECG waveform looks like –

The P, QRS, T sections are shown below –

The nature and amplitudes of the P, QRS, T and U greatly helps to make the diagnosis of the various types o

heart diseases. The typical Values of the above complex for the normal healthy heart are –

P

Q

R

S

T

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2

Chapter 2

Project Specifications

•

Differential inputs

• Adjustable gain: 200 - 2,000

• Band pass filter • fc1 = 0.01 to 0.05 Hz

• fc2 = 250 to 750 Hz

• Minimize 60 Hz noise

Chapter 3

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3

System Design Considerations

3.1 Sources of Interference –

There are many external agencies that can potentially hamper the normal operation of ECG amplifier cir

Since ECG operates on a very small signal strength (of the order of mill volts). These sources of interfereshould be taken care of before they enter into the high gain areas of the systems. Most commonly obser

sources of noise/ interference are mentioned below.

1) Ground loops – Patients are connected to multiple pieces of equipment; each has a ground (power

or common room ground wire). If more that one instrument has a ground electrode connected to

patient, a ground loop exists. Power line ground can be different for each item of equipment, sencurrent through the patient and introducing common-mode noise.

2)

Unwanted voltage transients are possible in the circuits due to patient movement and Electrstimulation signals, like defibrillation it is possible that because of these transients the amplifier

saturate.

3) Magnetic Coupling is possible into the circuit because of power lines, Transformers and ballast

fluorescent lights

4) Electromagnetic radiation due to patient leads becoming antennas, especially if detached. Varsources are responsible for this kind of interference, most commonly, Radio, Television, Ra

Research equipments, electrosurgical devices, Arching fluorescent lights.

5)

Coupling of 60 Hz power line noise is the most commonly observed artifact due to nearby power liThe interference is possible through the air generated effective capacitance between instrument si

lines and the supply lines.

6) Electrostatic discharge due to electrosurgical equipment or leads shorted to high voltage by hosp

personnel.

3.2 Interference Elimination Strategies –

1) 60 Hz Notch Filter Design – The notch filter stage is implemented at the early stages of the ampl

system to prevent coupling and amplification of the noise. The notch filter eliminates only 6frequencies and allows all the other frequencies to the next stage.

2) Stage gains - Since the order of the input signal amplitude is mill volts, the gain of the systemapproximately of the order of 1000. Instead of amplifying the signal at a single stage, the signa

gradually amplified through varies stages of system. Also this ensures the high bandwidth of

system. (The gain Bandwidth product of OP-AMP is constant so more gain less frequency response)

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3) Instrumentation Amplifier Design – To avoid loading of a weak ECG signal, the first stage ofsystem is instrumentation amplifier which provides very high input impendence. Also the signa

amplified to a certain extent at this stage. This stage precedes the notch filter stage.

4) Integrator at the input stage – Small DC offset may be present at the input which also may goamplifying in the system causing amplifier saturation. To avoid this, the output of the first stage

instrumentation amplifier is fed to an integrator circuit. The output of the integrator is connected to

reference of the instrumentation amplifier. So the DC value is integrated through the integrator shifts the reference. Since the last stage of instrumentation amplifier is differential amplifier

eliminates the DC voltage.

5) Twisted pair cables – All the cables are essentially twisted pair cables to avoid the inductive eff

and magnetic interference.

6) Decoupling capacitors – Although a highly regulated DC supply is used for this application, sm

imperfections in the supply are possible. To avoid these imperfections affecting the operation of

amplifier, a small value capacitor (0.01uF) called as decoupling capacitors are connected betw

supply and ground and placed very close to every IC.

7) Band Pass filter Design – The usual range of frequencies of ECG amplifier is 0.05 – 750Hz. So

order band pass filter is designed to accommodate these frequencies. Also the High Filter precedeslow pass frequency filter.

8) Amplifier Protection – Voltage limiting devices on each input lead are used to protect the equipm

from high electrostatic discharge due to improper handling of the instrument. Usually the paradiodes are used at the input stage. So the diodes clip any voltage of the magnitude more than abso

value 0.7V. Since the signal range of the ECG signal (1 mV) is very less than threshold voltage of

diodes, the diodes don’t interfere with the ECG signals.

3.3 Circuit Design –

A) Stage 1 – Instrumentation Amplifier with the integrator feedback.

Features –

Ø Provides high impedance to the inputs to avoid loading.Ø Amplifies the signal with the gain 18 (acts as a preamplifier).

Ø Eliminates the DC offset voltage with the help of feedback integrator

B) Stage 2 – Notch filter followed by unity gain amplifier.

Features –

Ø Eliminates the 60 Hz frequency components present in the input stages.Ø The unity gain amplifier/ buffer provide highest bandwidth to the signal also avoids

loading.

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C) Stage 3 – High Pass filter

Features –

Ø This is the first stage of the band pass filter. Provides the cut off frequency 0.05 Hz.

the frequencies above 0.05 Hz are passed from the stage.

Ø This is second order filter so provides sharper cutoff at the pass band. Also this filter ithe type Sallen Key which is famous for their robustness.

Ø This stage also provides the gain of 2 to the signal.

D) Stage 4 – Low Pass filter –

Features –

Ø This is the second stage of the band pass filter. Provides the cut off frequency of 750

All the frequencies below 750 Hz are passed from the stage.

Ø This is second order filter so provides sharper cutoff at the pass band. Also this filter ithe type Sallen Key which is famous for their robustness.

Ø This stage also provides the gain of 2 to the signal.

E) Stage 5 – Final Stage – Variable Gain amplifier –

Features –

Ø This is the final stage of the ECG amplifier circuit. It is a simple non inverting ampli

providing the adjustable gain of 2 – 9.

3.4 Circuit Schematic –

Stage 1 : Instrumentation Amplifier Stage - Gain 18

X1

X2

X3

2.632K

Rg

25 K

R1

N1

N2

25 K

R2

N2

N1

N2

N1

60 K

R3

60 K

R4

60 K

R5

60 K

R6

N3

N1

N2

7

V1

Vinstru

Vinput

7 V2

0

Vin

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Stage 2 : Notch Filter - 60 Hz Gain 1

Stage 3 : Sallen Key High Pass Filter : fc = 0.05 Hz Part 1

0.34uC1

0.034u

C2

0.068u

C3

39.01K R7 39.01K R8

19.5K

R9

N5N3

X4

N1

N2

1K

R13

1K

R12

N6N5

300K

R11

300K R10

10uC510uC4

X5 N1

N2

VnotchVbuffer

VgainHP1Vhighpass1

Stage 4 : Sallen Key Low Pass filter : fc = 750 Hz Part 1

Stage 3 : Sallen Key High Pass Filter : fc = 0.05 Hz Part 2

0.01u

C6

10 K

R14

X6

0.01u

C7

10 K

R15

N1

N2

1K

R16

3K

R17

N7N8

VgainLow pass1VgainLP1

1K

R22

1K

R23

N7N6

300K

R24

300K R25

10 uC910 uC10

X9 N12

N13VgainHP2

Vhighpass2

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Stage 4 : Sallen Key Low Pass filter : fc = 750 Hz Part

VgainLP2VgainLowpass2

N9N8

3K

R29

1K

R28

N11

N10

10K

R27

0.01u

C12

X10

10K

R26

0.01u

C11

Stage 5 : F inal Amplification Stage : G ain 2 - 10

Integra tor for Refera nce of INA118

X7

1K R18

1K

R19

8K

R20

N2

N1

N9

Vfinal

X8

1Meg

R21

N3

1uC8

N1

N2Vintegrate

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8

Chapter 4

Testing and Results

4.1 Simulation using B2spice – Simulation Graph Attached.

4.2 Debugging using Signal Generator –

The circuit performance is measured using sinusoidal signal generator. The signal generator was fixe

0.01V and the frequency of the waveform was varied and the filter responses were measured. The cutfrequencies were verified. After all the circuit verification, the circuit was tested with the ECG wavef

generator. The output of the circuit is given in the next section.

The circuit was tested step by step as follows.

Stages Instru.

Amp.

Notch

Filter

Integrator 4th Order HP Filter

f c = 0.05Hz

4th Order LP Filter

f c = 710Hz

High Gain

Amplifier

Comments Result

Specs. 60 Hz High

Pass

Stage 1

High

Pass

Stage 2

Low

Pass

Stage 1

Low

Pass

Stage 2

Non

inverting

Gain of

the stage

18 1 2 2 2 2 1 - 9

Debugging

Step 1 In

test

Not

used

Not used3 Not

used

Not

used

Not

used

Not

used

Not used Sine wave given

as an input and

output was tested

on the

oscilloscope.

Faithful

amplificatio

was found.

Test

Successful Step 2 In

test

By

passed1

Not used3 In test Not

used

Not

used

Not

used

Not used The oscilloscope

is connected and

frequency

response gain

was observed.

Faithful

amplificatio

Test

Successful

Step 3 In

test

By

passed1

Not used3 In test In test Not

used

Not

used

Not used Same as above Test

Successful

Step 4 In

test

By

passed1

Not used3 In test In test In test Not

used

Not used The frequency of

the input is

varied around the

cut off and the

output is

observed withthe oscilloscope

The cut off

frequency2

was adjuste

720 Hz.

Test

Successful

Step 4 In

test

By

passed1

Not used3 In test In test In test In test Not used Same as above Test

Successful

Step 5

(Entire

System)

In

test

By

passed1

Not used3 In test In test In test In test In test The input of the

circuit was

connected to

ECG waveform

generator and the

final output was

observed.

ECG

waveform

was seen on

the

oscilloscop

Test

Successful

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4.3 ECG Amplifier Output –

Figure 4.1

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Chapter 5

Design Issues / Implementation / Practical Considerations

1) Lot of Gain reduction was found at the Notch Filter Stage. So I decided to bypass it till the final ou

was seen. Later I found that there is not 60Hz interference present in the operation of circuit. So forsake of simplicity and ease of debugging of the prototype, I disconnected this stage from the opera

permanently. However this stage is present in all the professional circuits so for final design it has t

implemented.2) To test the cutoff frequency, the output on the oscilloscope was observed and the frequency of the in

is varied. The output divisions were observed till they reduce by the factor 0.707. The frequency at

point was found out and labeled as cut – off frequency. In the first testing, it was found thattheoretically calculated resistances didn’t give the targeted frequency values so the using extrapolat

the resistance values are chosen.

3) I didn’t implement integrator stage. Instead of that I connected reference point to the ground.

4) I didn’t use Sallen – Key filters in this design. I have used forth order High pass and low pass pas

filters with gain. The circuit gave the good results so I didn’t go for Sallen – Key filters which wohad led to more complicated wiring due to feedback.

5) Since the circuit was built on the bread board, I draw the layout of the circuit in such a way that thwould be least interference of the cables, components. I have laid down the cables as shown in

following figure. As we can see the circuit is very “clean” and very easy debug. This the prime rea

for not implementing integrator and notch filter stages.

Figure 5.1

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6) The ground lines and supply lines are tracked through out the board. As you can see in the diagram

all the blue lines indicate ground signal. The middle two red lines implement +Vcc and the extreme

lines indicate – Vcc. All the filter stages are implemented sequentially as seen on the top. T

implementation greatly reduced wiring in the circuit. This lead to easy implementation and debugginthe circuit. The middle two OP – AMPs in the bottom were used to implement Notch Filter

integrator stages, but finally weren’t included in the analysis.

7) Decoupling capacitances were added very close to each IC to suppress the supply imperfections8) However I should mention here, there can be potential problem in the operation, as you can see th

pins (pin 4) of OP – AMPs is grounded at the different points. This can lead to common impeda

interference. Actually there must be “Star ” ground for these ICs.

Summery of the Design

The circuit can potentially give wrong results due to absence of Notch filter; integrator to eliminate

offset and Star ground. Also since the components are not soldered and just placed in the breadboard, there

be significant contact resistances between the connections. The presence of these resistances may alter

frequency response of the filters and gain. Also the presence of high tolerance resistance is one point. Usuthese Medical Instrumentation Applications are implemented using Metal film precision resistors and trim p

These all can be potential bugs in the circuit, but I would like to mention famous quote saying “Known bugsnot problems!!” These issues will definitely be considered in the actual PCB design.

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Chapter 6

Component Listing

Sr.No. Component Value / Brand Name Quantity1 Instrumentation Amplifier Burr – Brown INA118 1

2 Operational Amplifiers National LM741 5

3 Resistances 1K Ω

300K Ω

10K Ω 8K Ω

8

4

41

4 Capacitances 0.01μF

10 μF

10

4

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Chapter 7

References

1)

Medical Instrumentation – Application and Design by J.G. Webster (3

rd

Edition)2) ECE 525 Lecture Notes by Prof. Dr. T. Nagle, North Carolina State University.3) http://www.math.princeton.edu/~simas/ecg.html#results

4) http://www.ecglibrary.com/norm.html

5) Research Paper on Electrocardiogram – MIT Department of Electrical Engineering and computer Scienc

http://www.math.princeton.edu/~simas/ecg.html#results

http://www.ecglibrary.com/norm.html

http://www.ecglibrary.com/norm.html

http://www.math.princeton.edu/~simas/ecg.html#results

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