ENT 412 Bioelectrical Instrumentation Design_4

7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4

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ENT 412 BIOELECTRICAL

INSTRUMENTATION DESIGN

BIOELECTRODES


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A BRIEF H ISTORY OF BIOELECTRODES Electrochemistry studies on electrode polarization

Electrode polarization is an interfacial phenomenonoccurring at the electrode-electrolyte interface

Research started from 1826 Current research: Extensive work on tissue

impedance measurements was done by Schwancommencing in 1951

Schwan also engaged in extensive studies onpolarization phenomena involving platinumelectrodes, including platinum black electrodes overboth the linear and non-linear range

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SOURCES OF BIOELECTRIC SIGNALS Endogenous and Exogenous signals

Endo - arise from natural physiological processes andare measured within or on living creatures

Exo- applied from without (generally noninvasively)to measure internal structures and parameters

Bioelectric signals arise from the time-varyingtransmembrane potentials seen in nerve cells(neuron action potentials and generator

potentials) and in muscle cells

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EMG EMG recording is used to diagnose some causes of

muscle weakness or paralysis, muscle or motorproblems such as tremor or twitching, motor nervedamage from injury or osteoarthritis, and pathologies

affecting motor end plates Carried out on of skeletal muscles and superficial

muscles

A skeletal muscle fiber action potential propagates at3 to 5 m/sec; its duration is 2 to 15 msec, depending

on the muscle, and it swings from a resting value ofapproximately -85 mV to a peak of approximately +30mV. At the skin surface, it appears as a triphasicspike of 20- to 2000-mV peak amplitude (Guyton,1991)

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EMG amplifier gains are typically X1000 andtheir bandwidths reflect the transient nature of

the single motor units (SMU) action potentials -reactively coupled with low and high -3-dBfrequencies of 100 and 3 kHz, respectively

EMGs can be viewed in the time domain (most

useful when single fibers or SMUs are beingrecorded), in the frequency domain (the FFT istaken from an entire, surface-recorded EMGburst under standard conditions), or in the timefrequency (TF) domain

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ECG

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The QRS spike in the ECG is seen to be associated

with the rapid rate of depolarization of ventricularmuscle just preceding its contraction. The P wave iscaused by atrial depolarization and the T wave isassociated with ventricular muscle repolarization

ECG QRS spike can range from a 400-mV to 2.5-mV

peak - the gain required for ECG amplification isapproximately 103

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EEGThe largest EEG potentials recorded on the scalp

are approximately150 mV at peak

The standard 10 to 20 EEG electrode array uses

19 electrodes; some electrode arrays used inbrain research use 128 electrodes

EEG amplifiers must work with low-frequency,low amplitude signals; consequently, they mustbe low noise types with low 1/f noise spectrums.

EEG amplifiers can be reactively coupled; their -3-dB frequencies should beabout 0.2 and 100 Hz.Amplifier midband gain needs to be on the orderof 104 to 105 10


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The most noteworthy features of biopotentials are

Small amplitudes (10 mV to 10 mV)

Low frequency range of signals (dc to several hundredhertz)

The most noteworthy problems of such acquisitionsare

Presence of biological interference (from skin,electrodes, motion, etc.),

Noise from environmental sources (power line, radiofrequency, electromagnetic, etc.).

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PRINCIPLES OF BIOPOTENTIAL

MEASUREMENTS Electrode design and its attachment suited to the

application;

Amplifier circuit design for suitable amplificationof the signal and rejection of noise andinterference;

Good measurement practices to mitigateartifacts, noise, and interference.

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ELECTRODES FOR BIOPOTENTIAL

RECORDINGS SilverSilver Chloride Electrodes

consists of a highly conductive metal, silver,interfaced to its salt, silver chloride, and connectedvia an electrolytic gel to the human body

design to produce the lowest and most stable junctionpotentials - J unction potentials are the result of thedissimilar electrolytic interfaces, and are a serioussource of electrode-based motion artifacts

additionally, an electrolytic gel typically based on

sodium or potassium chloride is applied to theelectrode

A gel concentration in the order of 0.1M (molarconcentration) results in a good conductivity and lowjunction potential without causing skin irritation

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Reusable silversilver chloride electrodes are made ofsilver disks coated electrolytically by silver chloride,or, alternatively, particles of silver and silver chlorideare sintered together to form the metallic structure ofthe electrode.

suited for acute studies or basic researchinvestigations

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Disposable electrodes are made similarly, althoughthe use of silver may be minimized (for example,thesnap-on button itself may be silver coated andchlorided).

To allow for a secure attachment, a large foam padattaches the electrode body with adhesive coating onone side.

Suited for ambulatory or long term use. 16


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Gold Electrodes

have the advantages of high conductivity andinertness desirable in reusable electrodes

commonly used in EEG recordings

Small reusable electrodes are designed so that theycan be securely attached to the scalp

The electrode body is also shaped to make a recessedspace for electrolytic gel, which can be appliedthrough a hole in the electrode body

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The electrodes are attached in hair-free areas by useof a strong adhesive

Disadvantagesof using gold electrodes over silversilver chloride electrodes - greater expense, higher

junction potentials, and greater susceptibility tomotion artifacts

Advantages - maintain low impedance, inert andreusable, and good for short-term recordings as longas a highly conductive gel is applied and they are

attached securely

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Conductive Polymer Electrodes

Certain polymeric materials

have adhesive properties andby attaching monovalent

metal ions can be made conductive

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The polymer is attached to a metallic backing madeof silver or aluminum foil, which allows electriccontact to external instrumentation

This electrode does not need additional adhesive orelectrolytic gel

The conductive polymeric electrode performsadequately as long as its relatively higher resistivity

(over metallic electrodes) and greater likelihood ofgenerating artifacts are acceptable

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Needle Electrodes

comprise a small class of invasive electrodes, usedwhen it is absolutely essential to record from theorgan itself

The most common application is in recording frommuscles or muscle fibers

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A metallic, typically steel, wire is delivered via aneedle inserted at the site of the muscle fiber. Thewire is hooked and hence fastens to the muscle fiber,even as the needle is removed. Small signals such as

motor unit potentials can be recorded in this manner

use is limited to only highly specialized andsupervised clinical or research applications

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ELECTRIC CHARACTERISTICS

The electric characteristics of biopotentialelectrodes are generally nonlinear and a functionof the current density at their surface

electrodes can be represented by an equivalentcircuit

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Rd andCdar e componen tsthat represent theimpedance associated with the electrodeelectrolyte interface and polarization at thisinterface.

Rs i s the ser i es resi stance associ at ed w i th

i n ter faci al effects and the r esi stance of the

electrodematerials themselves

The battery Ehc r epr esen ts th e hal f-cel l poten ti al

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An example of biopotential electrodeimpedance as a function of frequency.Characteristic frequencieswill be somewhat different for electrodedifferent geometries and materials.


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The Effect of Electrode Properties on Electrode Impedance


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BIOPOTENTIAL

AMPLIFIERS

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The Instrumentation Amplifier

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The instrumentation amplifier. This amplifier has a very high input impedance, high CMRR, and adifferential gain set by the resistors in the two amplifier stages. The gain of the first stage (amplifiers A1 and A2)is 1 +2R2/R1, the second stage (amplifier A3) is R4/R3, and the third stage (amplifier A4) is 1 + R7/R6. Thelower cornerfrequency is 1/(2R5C1) and the upper corner frequency is 1/(2R7C2). The variable resistor R isadjusted to maximize the CMRR. Electrodes E1 and E2 are the recording electrodes while E3 is the referenceor the ground electrode.


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The key design component of all biopotentialamplifiers is the instrumentation amplifier

This design results in the desired differentialgain distributed over two stages of the amplifier

I t also achieves a very high input resistance as aresult of the noninverting amplifier front end

I t exhibits a very high CMRR as a result of thedifferential first stage followed by a second-stagedifferential amplifier - The CMRR is enhanced byadjusting one of the matching resistors and byselecting high CMRR op amps

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ECG AMPLIFIERS

Active filters with a lower corner frequency of0.05 Hz and an upper corner frequency of 100 Hzare also typically added

leakage from the amplifier is required to be belowthe safety standard limit of 10 mA

safety of the patient is achieved by providingelectrical isolation from the power line and theearth ground, which prevents passage of leakage

current from the instrument to the patient undernormal conditions or under reasonable failureconditions

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Electrical isolation is achieved by usingtransformer or optical coupling components

In use with defib - the amplifier circuit must be

protected against the high defibrillation voltagesand must be augmented by circuit componentssuch as current-limiting resistors, voltage-limiting diodes, and spark gaps

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EEG AMPLIFIERS

The distinguishing feature of an EEG amplifier isthat it must amplify very small signals

all components of the amplifier must have a very

low thermal noise and in particular low electronic(voltage and current) noise at the front end of theamplifier

EEG amplifiers used in clinical applications mustbe electrically isolated and protected against high

defibrillation voltages

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EMG AMPLIFIERS

EMG amplifiers are often used in theinvestigation of muscle performance,neuromuscular diseases, and in building certainpowered or smart prostheses - enhancedamplifier bandwidth suffices

postprocessing circuits are almost always needed

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CIRCUIT ENHANCEMENTS

These enhancements include circuits for reducingelectric interference, filtering noise, reduction ofartifacts, electrical isolation of the amplifier, andelectrical protection of the circuit againstdefibrillation shocks

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Electrical Interference Reduction

Sources of interference include induced signals frompower lines and electric wiring; RF fromtransmitters, electric motors, and other appliances;magnetically induced currents in lead wires; and soon

Interference induced on the body common to the

biopotential sensing electrodes is called the commonmode interference (as distinguished from thebiopotential that is differential to the sensingelectrodes)

The common mode interference is principally rejected

by a differential or instrumentation amplifier with ahigh CMRR. Further improvement is possible by useof the driven right leg circuit.

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Electrical Interference Reduction

The driven right leg circuit employs the clever idea ofnegative feedback of the common mode signal into this lead.The common mode signal is sensed from the first stage ofthe instrumentation amplifier, amplified and inverted, andfed back into the right leg lead

At this stage the common mode signal is reduced to(idR0)/ (1 + 2R2/ R1)

The driven right leg circuit along with a high CMRR of theamplifier and filtering permit very high quality biopotentialmeasurements

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The schematic on the left shows electric interference induced by thedisplacement current id fr om the power l i ne. Thi s cur r ent f lows in to thegroun d el ectr odelead generating common-mode voltageVc. The dr i venr i ght l eg ci r cui t on th e r i ght uses negati ve feedback i nt o th e

right leg electrode to reduce the effective common-mode voltage.


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Filtering filtering at the front end of the amplifier and limiting

the bandwidth of the biopotential amplifier canfurther help to reduce the interference

Small inductors or ferrite beads in the lead wireshelp to block very high frequency electromagneticinterference

Small capacitors between each electrode lead andground filter the RF interference

use of high-pass filtering in the early stages ofamplification is recommended - dc potentials arisingat the electrodeskin interface

Low-pass filtering at several stages of amplification

is recommended to attenuate residual RFinterference as well as muscle signal interference

a 50 or 60 Hz notch filter to remove the power lineinterference

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Amplifier front end filters T1: RF choke; R0

andC0: RF fi l ter ; R1 and C1: high-pass fi l ter ; R2and C2: low-pass fi l ter .


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Notch filter for power line interference

(50 or 60 Hz): twin T notch filter inwhich notch frequency is governed byR1,R2, R3, C1, C2, and C3, and notch

tuning byR4.


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Artifact Reduction

computerized processing may be necessary to identifyan artifact and delete it from display and processing

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Baseline restoration circuit: the high-

pass filter capacitor C1 i s di scha r ged byfi el d effect t r ansi stor

F when activated manually orautomatically by a baseline restorationpulse.


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Electrical Isolation

Electrical isolation limits the possibility of thepassage of any leakage current from the instrumentin use to the patient

patient safety must be ensured by electrical isolationto reduce the prospect of leakage of current from any

other sensor or instrument attached to the patient tothe Earth ground of the instrument being tested

Electrical isolation can be done electrically byinserting a transformer in the signal path or opticallyby introducing an optical coupler

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Electrical isolation: transformercoupled using the transformer T (top) or optical using thediode D and the photodetector P (bottom). Note that theisolator separates circuit common on the amplifier sidefrom the Earth ground on the output side.


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Defibrillation Protection

Biopotential-measuring instruments can encountervery high voltages, such as those from electricdefibrillators, that can damage the instrument

Therefore, the front end of the biopotentialinstrument must be designed to withstand these highvoltages

Use of resistors in the input leads can limit the

current in the lead and the instrument.

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Protection against high voltages is achieved by theuse of diodes or Zener diodes. These componentsconduct at 0.7 V (diode conduction voltage) or 10 to15 V (depending on the Zener diode breakdown

voltage), thus protecting the sensitive amplifiercomponents

As a final line of protection, the isolation components(optical isolator or transformer) must be protected by

a spark gap that activates at several thousand volts.The spark gap ensures that the defibrillation pulsedoes not breach the isolation.

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Electrical protection circuit: resistanceR l im i t sth e cur r ent , rever se-biased di odes D l im i t t he i nput

vol tage, and th e spar k gap Sprotects againstdefibrillation pulse-related breakdown of theisolation transformer T


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MEASUREMENT PRACTICES

Electrode use

Skin Preparation

Reduction of environmental interference

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CONCLUSION

Biopotential source presents its own distinct challenge interms of electrode interface, amplifier design, pre- orpostprocessing, and practical implementation and usage

ECG signals can be best acquired using AgAgCl electrodes,

although good experimental/clinical practice is needed toreduce biological and environmental interference. Furthercircuit protection and isolation are necessary in clinical usage

EEG signals are distinguishable by their very low amplitude,and hence EEG electrodes must be securely attached via avery small electrodeskin resistance and the amplifier must

exhibit exceptionally low noise

For EMG acquisition, electrodes are needed that can beattached for long periods of time to the muscle groups understudy. The EMG signal inevitably needs postprocessing, suchas integration, to derive a measure of muscle activity

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ENT 412 Bioelectrical Instrumentation Design_4