Upload
rallapallisrinivas
View
220
Download
0
Embed Size (px)
Citation preview
7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4
1/49
ENT 412 BIOELECTRICAL
INSTRUMENTATION DESIGN
BIOELECTRODES
7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4
2/49
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
2
7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4
3/49
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
3
7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4
4/49
4
7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4
5/49
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)
5
7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4
6/49
6
7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4
7/49
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
7
7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4
8/49
ECG
8
7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4
9/49
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
9
7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4
10/49
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
7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4
11/49
11
7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4
12/49
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.).
12
7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4
13/49
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.
13
7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4
14/49
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
14
7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4
15/49
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
15
7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4
16/49
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
7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4
17/49
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
17
7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4
18/49
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
18
7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4
19/49
Conductive Polymer Electrodes
Certain polymeric materials
have adhesive properties andby attaching monovalent
metal ions can be made conductive
19
7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4
20/49
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
20
7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4
21/49
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
21
7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4
22/49
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
22
7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4
23/49
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
23
7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4
24/49
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
24
7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4
25/49
25
An example of biopotential electrodeimpedance as a function of frequency.Characteristic frequencieswill be somewhat different for electrodedifferent geometries and materials.
7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4
26/49
26
The Effect of Electrode Properties on Electrode Impedance
7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4
27/49
BIOPOTENTIAL
AMPLIFIERS
27
7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4
28/49
The Instrumentation Amplifier
28
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.
7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4
29/49
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
29
7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4
30/49
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
30
7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4
31/49
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
31
7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4
32/49
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
32
7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4
33/49
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
33
7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4
34/49
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
34
7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4
35/49
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.
35
7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4
36/49
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
36
7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4
37/49
37
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.
7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4
38/49
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
38
7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4
39/49
39
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 .
7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4
40/49
40
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.
7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4
41/49
Artifact Reduction
computerized processing may be necessary to identifyan artifact and delete it from display and processing
41
7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4
42/49
42
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.
7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4
43/49
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
43
7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4
44/49
44
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.
7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4
45/49
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.
45
7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4
46/49
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.
46
7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4
47/49
47
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
7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4
48/49
MEASUREMENT PRACTICES
Electrode use
Skin Preparation
Reduction of environmental interference
48
7/28/2019 ENT 412 Bioelectrical Instrumentation Design_4
49/49
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
49