69647_Origin of Bio-Potentials (1)

7/31/2019 69647_Origin of Bio-Potentials (1)

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Bio-Potential Amplifiers


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Basic Requirements for Amplifiers

Type of amplification: Voltage Amplification Current Amplification

High input impedance ( 10 MW) why? Isolation and protection circuits why? Low output impedance why?

High common mode rejection ration why? The appropriate frequency spectrum, SNR, gain, Calibration input to calibrate the amplifier


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Example of Bio-Potential AmplifierECG Amplifier

Origins of the electrocardiogram Blood Cycle

The cardiac vector The ECG waveform Indicator of a good ECG

12-lead electrocardiography


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Blood Cycle of the Heart

Objectives of the cycle: Provide Oxygenated blood to all body cells Remove Carbon-Dioxide accumulating in cells

Two Blood Cycles simultaneously Pulmonary cycle add oxygen and remove Co 2 Blood Cycle carry oxygen to body cells


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The Heart Beat (Cardiac cycle)

Two Phases1. Two atriums contract and two ventricles relax2. Two ventricles contract and two atriums relax

3. Four chambers relax Terminology:

Systolic phase contraction

Diastolic phase relaxation Control is done via an independent nervous

system in the heart though electrical signals


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Conduction mechanismof the heart


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Sequence of Control Signals

SA node (heart pace maker) initiate signal toatriums causing it to contract blood flowsthrough valves to ventricles

Impulse flows till it reaches AV node AV node fires a signal along the bundle of his

and ventricles contract in top to bottom fashion

to push the blood out of the heart http://en.wikipedia.org/wiki/File:ECG_principle_slow.gif#file
http://en.wikipedia.org/wiki/File:ECG_principle_slow.gifhttp://en.wikipedia.org/wiki/File:ECG_principle_slow.gifhttp://en.wikipedia.org/wiki/File:ECG_principle_slow.gifhttp://en.wikipedia.org/wiki/File:ECG_principle_slow.gif


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Components of ECG

The ECG is drawn against time. It contains elements indicating

the temporal relationshipbetween different actions takingplace during a cardiac cycle.

The respective amplitudes of these elements and their

respective positions are studied

Picture from wikipedia.com
http://upload.wikimedia.org/wikipedia/commons/9/9e/SinusRhythmLabels.svg


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ECG Amplifiers

The electrical activity of theheart can be modeled as anElectric dipole in the thorax

This dipole is representedwith a vector that varies inamplitude and direction(cardiac vector)

Defined lead vectors: unitvectors with fixed orientation

Vector can be observed fromdifferent directions andangles


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The electrocardiogram is measured by puttingelectrodes on specific locations on the body. The electrodes used, their respective wires and

resistors for measuring the ECG from aparticular direction is called the lead

Measurement in Frontal Plane Bipolar limb leads, I, II, III Unipolar limb leads, aV R, aV L, aV F

Precordial (chest) leads for transverse plane(V1-V6)

12-Lead Electrocardiography


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Augmentedunipolar limb leads

33% increase in voltagemeasured by disconnectingthe measuring electrodefrom the Wilsons terminal

without affecting thedirection of the lead vector.Three new configurationsthus obtained as aVR, aVLand aVF. a stands foraugmented.A resistance of R/2 isadded to balance the inputimpedances seen by both

inputs of the diff. amplifier.


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Homework 1Bio-Potential Amplifiers and ECG leads

1. Explain why Bio-Potential Amplifiers need to have the followingrequirements:

High Input Impedance Low output impedance Frequency response appropriate to the signal Isolation Circuit

2. Show that the voltage at wilson central point (figure 6.4) is the averageof the voltages at each node

3. Show that voltage at the augmented lead shown in figure 6.5 increasesthe output voltage and calculate this increase

Notes:

Homework due at beginning of Tue Oct 20th

class Late submission are subject to 10% decrease for everyday after the class No late submissions would be accepted after Sun Oct 24 th class


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Bio-Potential Amplifier II


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Electrode connections in uni-polar limbleads


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Electrode connections in unipolar chestleads


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Voltage and frequency ranges ofsome common bio-potential signals


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l k d f l f


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Block diagram of an earlier version ofan electrocardiograph


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F


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Frequencydistortion

The high f components of the signal are attenuatedyielding a rounding of sharp edges and droppingin the R-wave magnitude.This is a distortion in thesignal due to high f limitation.

A distortion due to low f limitation. The signal lookslike a differentiated one andthe stable baseline neededfor the clinical ECG is lost


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Artifacts from large electrical transients

Example: Defibrillator

S l i T i


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Solution: Transientprotection

A voltage protection scheme at the input of an ECG amplifier


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SolutionVoltage-limiting devices used for input

protection

Gas dischargetube (neonlight)

Back-to-back silicon zenerdiodes

Parallelsilicon diodes

Current-voltagecharacteristic

2-20 V 50-90 V

How do voltage limiting devices

protect the input of the amplifier?


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60-Hz power line andelectromyography interference

60-Hz power-line interference on the ECG

Electromyography interference on the ECGSolution : Proper filter design


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Effect of Frequency

Let go current Higher frequency,

less dangerous Why dont we

generateelectricity at high

frequencies?


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2-Generation

and effects ofmagneticfield

Protection through alternative path for the current

C Pi k d i h B d


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Currents Picked up in the Body1-Magnetic field pickup

SolutionTwisting lead wires togetherand keeping them close tobody minimizes interference

Lead wires for lead I making aclosed loop with patient and

ECG machine

Currents Picked up in the Body


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Currents Picked up in the Body2-Pickup due to displacement current flowing through the patient

GdbcmZ iv =

=in

cm B A Z Z Z

vvv 12

Z 1 and Z 2


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C t Pi k d i th B d


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Currents Picked up in the Body4-Electromagnetic interference

EM waves generated by Radar facilities X-ray machines Nearby transformers Radio waves

EM waves picked-up by patient and lead wires Demodulated by p-n junctions of transistors and/or

electrode-electrolyte interfaces Modulating audio signal appears as interference on

top of the ECG signal Solution: Can be eliminated by shunting the input

terminals of the ECG amplifier with a small capacitor(around 200pF)


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Solution: Electrostatic shielding


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Driven-right-leg circuit

Right leg connected to the output of an OPAMP instead of ground Why?1. Displacement current flows through output resistance instead of

body2. Patient un-grounded when high voltage appears between

patient and ground (R f and R o large values)


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Analysis of the driven-right-leg circuit


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Electrical and ComputerEngineering (Biomedical

45

Bio-Potential Amplifiers 3


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2-Generation

and effects ofmagneticfield

Protection achieved through alternative low resistancepaths for the current



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Currents Picked up in the Body1-Magnetic field pickup

SolutionTwisting lead wires togetherand keeping them close tobody minimizes interference

Lead wires for lead I making aclosed loop with patient and

ECG machine



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Currents Picked up in the Body2-Pickup due to displacement current flowing through the patient

GdbcmZ iv =

=in

cm B A Z Z Z

vvv 12

Z 1 and Z 2


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Currents Picked up in the Body3-Electrical field pickup by connecting wires and instrument

What is a Capacitor?Coupling between hot sideof the power line and leadwires

2211 Z i Z ivv d d B A =With i d1 i d2

)( 211 Z Z ivv d B A =

How to minimize the current picked up by lead wires and instrument?



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Currents Picked up in the Body4-Electromagnetic interference

EM waves generated by

Radar facilities X-ray machines Nearby transformers Radio waves

EM waves picked-up by patient and lead wires Demodulated by p-n junctions of transistors and/or

electrode-electrolyte interfaces Modulating audio signal appears as interference on

top of the ECG signal Solution: Can be eliminated by shunting the inputterminals of the ECG amplifier with a small capacitor(around 200pF)


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Solution: Electrostatic shielding


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Analysis of the driven-right-leg circuit


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0

2

=+ f o

a

cm

R

v

R

v

cma

f o v R

Rv

2=

od RLcm vi Rv +=

KCL at point x

i.e.

But

Therefore

d

a

f

RL

cm i R

R

R

v 21 +=

What is the effective resistance betweenright leg and ground?

Large transients Saturation choselarge R f and R o =~ 5 M

Regular operation want v cm as small aspossible large R f and small R a

Driven Right Leg Circuit


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Bio-Potential Amplifiers -4


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Example of a simple ECG amplifier


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Examples of Bio-Potential

Amlifiers

Bi di l Si l P


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Biomedical Signal ProcessorExamples

Cardiac Tachometers Electromyogram integrators Fetal electrocardiography Cardiac monitors Biotelemetry


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TachometersAveraging type


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EMG integrators

Switch

Absolute-valuecircuit

Monostablemultivibrator

Comparator

C

EMG

Integrator

+

R

P 1

v t

1

Counter

2

3


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EMG integrators


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Fetal electrocardiology

A t h i f i l ti f t l ECG f


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A technique for isolating fetal ECG frommaternal


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TelemetryFrequency modulation


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Digital landline telemetry system

ree-c anne me- v s onl i l d di l


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multiplexed radiotelemetrytransmitter

Example of outputwaveform from commutator


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Three-channelfrequency-divisionmultiplexed

radiotelemetrysystem

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69647_Origin of Bio-Potentials (1)