electronic measurements and instrumentation - 4

7/27/2019 Electronic Measurements and Instrumentation - 4

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EC-226 Electronic measurement andInstrumentation

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r n vasa ao r n vasa ao r n vasa ao r n vasa ao r n vasa ao r n vasa ao r n vasa ao r n vasa ao Dept. of ECE

Bapatla Engineering College ( Autonomous)


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Part 1DATA ACQUISITION SYSTEMS

Digital Data Acquisition System, Various ways of multiplexing,Computer controlled instrumentation.

BIO-MEDICAL MEASUREMENTS

Bioelectric signals (ECG,EMG,ERG,EOG) and electrodes.Elementary Principles of Electrocardiograph, Electromyograph,

Electroencephalograph.

T Srinivasa Rao Electronic Measurements and Instrumentation (EC-226)


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Data acquisition systems (DAS) interface between the real world of physicalparameters, which are analog, and the artificial world of digital computation andcontrol(digital systems are used widely because complex circuits are low cost,accurate, and relatively simple to implement. In addition, there is rapid growth in

the use of microcomputers to perform difficult digital control and measurementfunctions).The devices that perform the interfacing function between analog and digitalworlds are known as data converters.( ADC &DAC)

Data Acquisition Systems Basics

Besides the converters, data acquisition and distribution systems may employ oneor more of the following circuit functions:Transducers,Amplifiers,

Filters,Nonlinear analog functions,Analog multiplexers,Sample-holds.



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Digital Data Acquisition System



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The input to the system is a physical parameter such as temperature, pressure,flow, acceleration, and position, which are analog quantities is first converted intoan electrical signal by means of a transducer; once in electrical form, all furtherprocessing is done by electronic circuits.

An amplifier boosts the amplitude of the transducer output signal to a useful levelfor further processing. Transducer outputs may be microvolt or millivolt levelsignals, which are then amplified to 1 to 10V levels.The amplifier is frequently followed by a low-pass active filter that reduces high-

Data Acquisition System Functioning

frequency signal components, unwanted electrical interference noise, or electronicnoise from the signal.The processed analog signal next goes to an analog multiplexer, which switches

sequentially between a number of different analog input channels.

Each input is in turn connected to the output of the multiplexer for a specifiedperiod of time by the multiplexer switch. During this connection time, a sample-hold circuit acquires the signal voltage and then holds its value while an A/Dconverter converts the value into digital form. The resultant digital word goes to acomputer data bus or to the input of a digital circuit



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Data Distribution System



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Various ways of Multiplexing



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Computer Controlled Instrumentation

Transducer PC BasedController Transducer

OutputContolling

UnitTo Process

PhysicalVariable


MONITOR

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PRINTER


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Electrocardiograph (ECG)Electrocardiography (ECG) ( Greek: kardia , meaning heart) is a

transthoracic (across the thorax or chest) interpretation of the electrical activity ofthe heart over a period of time, as detected by electrodes attached to the surface ofthe skin and recorded by a device external to the body. The recording produced bythis noninvasive procedure is termed an electrocardiogram .

An ECG is used to measure the rate and regularity of heartbeats, as well asthe size and position of the chambers, the presence of any damage to the heart, andthe effects of drugs or devices used to regulate the heart.



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ECG Electrodes



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WorkingAn ECG is a way to measure and diagnose abnormal rhythms of the heart,particularly abnormal rhythms

caused by damage to the conductive tissue that carries electrical signals, or abnormal rhythms caused byelectrolyte imbalances.

The ECG can identify if the heart muscle has been damaged in specific areas, though not all areas of theheart are covered.

The ECG cannot reliably measure the pumping ability of the heart, for which ultrasound-based(echocardiography ) or nuclear medicine tests are used. It is possible for a human or other animal to be

in cardiac arrest , but still have a normal ECG signal (a condition known as pulseless electrical activity ).The ECG device detects and amplifies the tiny electrical changes on the skin that are caused whenthe heart muscle depolarizes during each heartbeat . At rest, each heart muscle cell has a negative charge,called the membrane potential, across its cell membrane . Decreasing this negative charge towards zero, viathe influx of the positive cations, Na + and Ca ++, is called depolarization, which activates the mechanisms in

. ,

wave of depolarisation that is triggered by the cells in the sinoatrial node , spreads out through the atrium ,passes through the atrioventricular node and then spreads all over the ventricles . This is detected as tinyrises and falls in the voltage between two electrodes placed either side of the heart which is displayed as awavy line either on a screen or on paper. This display indicates the overall rhythm of the heart andweaknesses in different parts of the heart muscle.

Usually, more than two electrodes are used, and they can be combined into a number of pairs (For

example: left arm (LA), right arm (RA) and left leg (LL) electrodes form the three pairs LA+RA, LA+LL, andRA+LL). The output from each pair is known as a lead . Each lead looks at the heart from a different angle.Different types of ECGs can be referred to by the number of leads that are recorded, for example 3-lead, 5-lead or 12-lead ECGs (sometimes simply "a 12-lead"). A 12-lead ECG is one in which 12 different electricalsignals are recorded at approximately the same time and will often be used as a one-off recording of an ECG,traditionally printed out as a paper copy. Three- and 5-lead ECGs tend to be monitored continuously andviewed only on the screen of an appropriate monitoring device, for example during an operation or whilstbeing transported in an ambulance. There may or may not be any permanent record of a 3- or 5-lead ECG,depending on the equipment used.T Srinivasa Rao Electronic Measurements and Instrumentation (EC-226)


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Rhythm Strip



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Electroencephalograph (EEG)EEG is the recording of electrical activity along the scalp. EEG measures voltagefluctuations resulting from ionic current flows within the neurons of the brain. Inclinical contexts, EEG refers to the recording of the brain's spontaneous electricalactivity over a short period of time, usually 2040 minutes, as recorded frommultiple electrodes placed on the scalp. Diagnostic applications generally focus onthe spectral content of EEG, that is, the type of neural oscillations that can beobserved in EEG signals.


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In an EEG, electrodes are placed on the scalp over multiple areas of the brain todetect and record patterns of electrical activity and check for abnormalities.EEG cannot be used to "read the mind," measure intelligence, or diagnose mentalillness.

EEG activity can be broken down into 4 distinct frequency bands:Beta activity > 13 Hz

Alpha activity 8 Hz-13 Hz

Theta activity 4 Hz-7 Hz

Delta activity < 4 Hz


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ElectrodesMetal Disk and Cup Electrodes

Disk and Cup electrodes are generally made of high purity tin,silver, gold or even surgical steel, or some combination of these(i.e. gold plated silver). They usually have a diameter that iswithin 4-10 mm as smaller than 4mm, or larger than 10mm, will

not make sufficient mechanical/electrical contact with the scalp.The application site is determined and prepared by sterilizingwith alcohol, using an abrasive to remove dead skin. Once theelectrode is secure, the cup is filled with a conductive gel whichaids conductivity. These electrodes can also be placed on other

T Srinivasa Rao

parts of the body to monitor skin potentials and filter these out,increasing the reliability of the readings.

Needle ElectrodesNeedle electrodes are sterilized, single-use needles placed under theskin. These electrodes are typically made of surgical grade steel andare inserted into the scalp after thorough disinfection of the insertionsite. The advantage of this particular type of electrode is fastapplication, perhaps being useful in emergency situations or oncomatose patients; however the invasive nature of the application andpain caused to the patient means that they are not commonly used.These electrodes also have a risk of infection and unfavorableelectrical behavior due to their shape. These electrodes are usually

used during intra-operative recordings as well as in association withintra-cranial electrode recording. Electronic Measurements and Instrumentation (EC-226)


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Electroencephalograph (EEG)



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Amplification



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Beta activity is a normal activity present when the eyes are open or closed. It tends tobe seen in the channels recorded from the centre or front of the head. Some drugs will increase

the amount of beta activity in the EEG.

p a ac v y s a so a norma ac v y w en presen n wa ng a u s. s ma n yseen in the channels recorded from the back of the head. It is fairly symmetrical and has anamplitude of 40 V to 100 V. It is only seen when the eyes are closed and should disappear orreduce in amplitude when the eyes are open.

Theta activity can be classed as both a normal and abnormal activity depending on theage and state of the patient. In adults it is normal if the patient is drowsy. However it can alsoindicate brain dysfunction if it is seen in a patient who is alert and awake. In younger patients,theta activity may be the main activity seen in channels recorded from the back and central areasof the head.T Srinivasa Rao Electronic Measurements and Instrumentation (EC-226)


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Delta activity is only normal in an adult patient if they are in a moderate to deep sleep.If it is seen at any other time it would indicate brain dysfunction. Abnormal activity may be seenin all or some channels depending on the underlying brain problem.



Spike and wave activity indicates a seizure disorder and may be seen in the EEGeven if the patient is not having an epileptic seizure. Other epileptic conditions may be

diagnosed if spikes or sharp waves are seen.

Triphasic waves are sometimes seen if the patient has severe liver or kidney disease

that is affecting brain function.


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Electromyograph (EMG)Electromyography, or EMG, involves testing the electrical activity of

muscles.Often, EMG testing is performed with another test that measures the

conducting function of nerves called nerve conduction study.Because both tests are often performed at the same office visit and by the

same personnel, the risks and procedures generally apply to both tests.

an electrical current. This electrical current is much weaker than the one inyour household wiring.

In some medical conditions the electrical activity of the muscles or nervesis not normal. Finding and describing these electrical properties inthe muscle or nerve may help doctors diagnose patient condition.

EMG may aid with the diagnosis of nerve compression or injury, nerve rootinjury and with other problems of the muscles or nerves.



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Electrodes



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Electromyograph (EMG)



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Electro-Oculogram (EOG)Electrooculography (EOG / E.O.G. ) is a technique for measuringthe resting potential of the retina. The resulting signal is called theelectrooculogram. The main applications arein ophthalmological diagnosis and in recording eye movements. Unlikethe electroretinogram, the EOG does not represent the response ton v ua v sua st mu .

Eye movement measurements: Usually, pairs of electrodes are placedeither above and below the eye or to the left and right of the eye. If the eyeis moved from the center position towards one electrode, this electrode

"sees" the positive side of the retina and the opposite electrode "sees" thenegative side of the retina. Consequently, a potential difference occursbetween the electrodes. Assuming that the resting potential is constant, therecorded potential is a measure for the eye position.



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There are a number of corneal electrodes that are in common use. Some are speculum structures that hold the eyeopen and have a contact lens with a wire ring that floats on the cornea supported by a small spring. Some versions usecarbon, wire or gold foil to record electrical activity. There are also cotton wick electrodes.

Each of these electrodes record large voltage responses directly from the cornea or sclera and each have advantagesand disadvantages. Burian speculum electrodes are used when possible. Sizes are available down to a size that fits in

the eye of most full-term babies


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Electroretinogram (ERG)The electroretinogram (ERG) is a

recording of electrical potentials(action potentials) that are generatedwithin the retina (typically in responseto a flash of light). It is recorded usingtwo electrodes, with one electrodeplaced on or close to the cornea of theeye whilst the other is placed on theforehead, cheek or earlobe.Electroretino ra h tests are

commonly performed after a subjecthas spent several minutes in completedarkness, thereby ensuring the lightflash is a significant stimulus.


Under these conditions the ERG response will be relatively large in amplitude (approximately500 V to 1mV), approximately 2 seconds in duration (depending on species) and will consist of several distinct components. In brief, the ERG may be separated into 4 waveform components (a,b, c and d waves). One electrode is placed on or close to the cornea of the eye. Several types of corneal electrodes are available for this application, including:

Contact lens with a steel or silver wire embedded in the inner surface

A piece of gold leaf tucked underneath the lower eye lidA skin electrode placed on the external surface of the lower eye lid .


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Electroretinogram


Electroretinography measures the electrical responses of various celltypes in the retina, including the photoreceptors (rods and cones), innerretinal cells (bipolar and amacrine cells), and the ganglion cells


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Electronic Measurements and Instrumentation (EC-226)T Srinivasa Rao

electronic measurements and instrumentation - 4

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