Biopotentials Electrodes and Amplifiers , Exam-2018

Department of Applied Physics and Electronic Engineering, University of Rajshahi

Bio electrodes: It is a class of sensors that transduce ionic conduction to electronic conduction. The usual purpose
of bio electrodes is to acquire medically significant bioelectric signal (as shown in fig-1) such as

1. ECG (electrocardiographic)
2. EEG (electroencephalographic)
3. EMG (electromyography)

Function of electrodes:

Microelectrodes: it is used to measure bioelectric

potential near or within single cell.
Skin surface electrodes: used to measure ECG, EEG and
EMG potentials from the surface of the skin.
Needle electrodes (Indwelling microelectrodes): It is used Figure 1: Bioelectric electrode and signal acquisition
to penetrate the skin to record EEG potential from brain or
EMG signal from group of muscle.

Electrode potential: The skin and other tissues of human are electrolytic and can be considered as electrolytic
solution. If a metallic electrode immersed in an electrolytic solution, the electrode will begin to discharge dome
metallic ions into the solution, while some ions in the solution start combining with metallic electrodes.

After a short while, a charge gradient builds up, creating a potential difference known
as electrode potential Ve (as shown in fig-2).

At interface, these reactions take place

Oxidizing reaction: metal electron + metal ions.

Reduction: electron + metal ions metal

Examples of electrode potential:

Figure 2: electrode
Potential formation

At the interface between electrode and electrolytic solutions, ions migrate and forms parallel layer of ions of
opposite charge. This is called electrode double layer and it is the source of Ve .

Materials for bioelectric electrode: Body fluids are corrosive to metal, so not all metals are used for bioelectric
sensing. Some are toxic to living tissues. For these reasons noble metals (Au, platinum), some tungsten alloys and
silver-silver chloride are used for bioelectric electrodes.

Skin impedance seen by electrodes:

For sweaty skin → 0.5 KΩ ; For dry skin → > 20 K

Problem skin, especially dry, scaly or diseased skin it may be ~ 500 KΩ.

Page 1 of 7
 Biopotentials Electrodes and Amplifiers , Exam-2018
Department of Applied Physics and Electronic Engineering, University of Rajshahi

Therefore, a skin can be treated as very high impedance voltage source. The high impedance of signal source
seriously influences the design of bio potential amplifier. Bio potential amplifier must have input impedance
equal to more than 10 times of source impedance (~ 5 MΩ). So, FET and op-amp are frequently used to design
bioelectric amplifier.

Surface electrodes: surface electrodes have been

designed for surface acquisition of bioelectric signals.
Two types of ECG electrodes are used in surface
acquisition-

Strap-on electrodes are 1 to 2 sq. inch brass plates that

are held in place by rubber straps (fig-3a). A (b)
conducting gel is used to reduce the impedance
between electrode and skin.
(a)
A suction-cup ECG electrode is used as a chest
electrode in short term ECG recording (fig-3b). Figure 3: Surface electrodes (a) strap-on and (b) suction-cup

Disposable column electrode: A disposable

column electrode consists of a silver-silver chloride
metal contact button at the top of a hollow column
that is filled with a conductive gel. This assembly
is held in a place by an adhesive-coated foam
rubber disk (fig-4).

The gel-filled column that holds the actual metallic Figure 4 : Disposal column electrode
electrode of the surface reduces the movement
artifact.

Needle electrodes: Needle type of ECG electrode is inserted

into the tissue immediately beneath the skin by puncturing the
skin at large oblique angle. It is only used for exceptionally
poor skin, especially anesthetized patients (fig-5a).
(a)
Indwelling electrodes. It is intended to be inserted into the
body. It is typically a tiny, exposed metallic contact at the end
of a long, insulated catheter (fig-5b).

In one application it is threaded through the patient’s vein to

the right side of the heart to measure the intra-cardiac ECG
signal.

Microelectrodes: It is an ultrafine device that is used to

measure bio potential at the cellular level as shown in fig-6.
There are two types of microelectrodes--
(b)
 Glass-metal microelectrode.
 Fluid-filled microelectrode Figure 5 : Needle (a) and indwelling electrodes

Page 2 of 7
 Biopotentials Electrodes and Amplifiers , Exam-2018
Department of Applied Physics and Electronic Engineering, University of Rajshahi

Metal Glass microelectrode:

The constructional details of a glass metal microelectrode is shown

in fig-6. The tip diameter of the electrode is of the order of micro
meter. It is used in the laboratory to measure the cell potential.
Microelectrode
Thin metal film coated microelectrode:

A thin film of precious metal is bonded to the outside of a drawn

glass microelectrode as shown in fig-7.. Then an insulating cover is
deposited on metal film but not on the top. This arrangement offers
lower impedance, infinite life and reproducible performance with
easy cleaning and maintenance.

Transducer: Transducers are devices which convert one form of Figure 6 : Microelectrode
energy into another. Because of many advantages of electric and
electronic measurement, non-electric phenomenon associated with
many physiological events is converted into electrical quantities.

Classification of Transducer:

1. By the process used to convert signal energy into electrical signal:

Active transducer: It can convert one form of energy directly into

another form. Example: Photovoltaic cell
Figure 7 : Glass micropipette coated with
Passive transducer: It requires energy to be put into it in order to metal film
translate changes due to the measurand. Example: Wheatstone bridge.

1. By the principle used: Half-effect transducer, piezo-electric transducer etc.

2. By application: flow transducer, pressure transducer, motion and temperature transducer etc.

Some performance characteristics of transducers: (static)

 Accuracy: It is the algebraic difference between indicated value and theoretical value.
 Precision: It refers to the degree of repeatability of the measurement.
 Resolution: It indicates the smallest measurable input increment.
 Sensitivity: It describe transfer ratio of output and input.
 Drift: It indicates a change of baseline (some output when input is zero) or of sensitivity with time,
temperature etc.
 Threshold: It is the smallest change in the measurand that will result in a measurable change in
output.(lower limit of measurand)
 Noise: Unwanted signal of output due to internal source or interference.
 Span: Operating range of transducer (example: bandwidth, temperature range etc.)

Page 3 of 7
 Biopotentials Electrodes and Amplifiers , Exam-2018
Department of Applied Physics and Electronic Engineering, University of Rajshahi

 Saturation: In general, output × input. Sometimes if input continues to increase, a point is reached where
the output no longer increases.

Strain gauge pressure transducer: (unbonded):

The transducer is based on the Wheatstone bridge principle.
It consists in mounting strain wires of two frames which may A
move with respect to each other as shown in fig-8.
B
The outer frame is fixed and the inner frame is connected to P
the diaphragm upon which the pressure acts, is movable. C
Pressure P on diaphragm stretches wires B and C and release D
A and D. These wires form a four arms of a Wheatstone
bridge. The equivalent circuit of strain gauge is shown in fig-
9.
Figure 8: Strain gauge pressure transducer
The resistance of Wheatstone bridge is changed if pressure is
changed at the input. A measurable output is obtained at output terminal
if P is changed.

Application: Used to measure blood pressure (direct measurement).

ECG (electrocardiograph):

The ECG is an instrument, which record the electrical activity of the E

heart. ECG provides valuable information about the wide range of
cardiac disorders. ECG is used in characterization laboratories, cardiac
core units and for routine diagnostic applications in cardiology.

A mv potential can be directly obtained by placing two electrodes and Figure 9: Equivalent circuit of strain
gauge pressure transducer
the frequency of this signal is 0.05 to 150 Hz. Negative feedback, high
gain amplifier is used for signal
recording.

ECG signal analysis:

First depolarization occurs during

normal sinus rhythm in the right atrium,
closely followed by the left atrium. So
the first electrical signal on a normal
ECG originates from the atria and is
known as the P wave.

There is then a short, physiological

delay as the atrioventricular (AV) node
slows the electrical depolarization
before it proceeds to the ventricles. This
Figure 10: ECG signal

Page 4 of 7
 Biopotentials Electrodes and Amplifiers , Exam-2018
Department of Applied Physics and Electronic Engineering, University of Rajshahi

delay is responsible for the PR interval, a short period where no electrical activity is seen on the ECG signal.

Depolarization of the ventricles results in usually the largest part of the ECG signal (because of the greater muscle
mass in the ventricles) and this is known as the QRS complex.

 The Q wave is the first initial downward or ‘negative’ deflection

 The R wave is then the next upward deflection (provided it crosses the isoelectric line and becomes
‘positive’)
 The S wave is then the next deflection downwards, provided it crosses the isoelectric line to become
briefly negative before returning to the isoelectric baseline.
In the case of the ventricles, there is also an electrical signal reflecting repolarization of the myocardium. This is
shown as the ST segment and the T wave.

Normal intervals

The recording of an ECG on standard paper allows the time taken for the various phases of electrical depolarization
to be measured, usually in milliseconds. There is a recognized normal range for such ‘intervals’:

 PR interval (measured from the beginning of the P wave to the first deflection of the QRS complex).
Normal range 120 – 200 ms.
 QRS duration (measured from first deflection of QRS complex to end of QRS complex at isoelectric
line). Normal range up to 120 ms.
 QT interval (measured from first deflection of QRS complex to end of T wave at isoelectric line).
Normal range up to 440 ms (though varies with heart rate and may be slightly longer in females)

EEG (electroencephalograph):

It is an instrument which records the electrical activity of the brain. By placing electrodes on the scalp, monitoring
the EEG has proven to be an effective method diagnosing may neurological illness and diseases (such as epilepsy,
tumor and problems associated with trauma).

The measurements given by an EEG are used to confirm or rule out various conditions, including: (applications)

 to monitor the nervous system of anaesthetized patient during operation

 seizure disorders (such as epilepsy)
 a head injury
 encephalitis (an inflammation of the brain)
 a brain tumor
 encephalopathy (a disease that causes brain dysfunction)
 memory problems
 sleep disorders
 stroke
 dementia
Page 5 of 7
 Biopotentials Electrodes and Amplifiers , Exam-2018
Department of Applied Physics and Electronic Engineering, University of Rajshahi

When someone is in a coma, an EEG may be performed to determine the level of brain activity. The test can also
be used to monitor activity during brain surgery.

Electromyography (EMG):

EMG is an instrument used for recording the electrical activity of muscles to determine whether the muscle is
contracting or not. Electromyography is a diagnostic procedure that evaluates the health condition of muscles and
the nerve cells that control them. These nerve cells are known as motor neurons. They transmit electrical signals
that cause muscles to contract and relax. An EMG translates these signals into graphs or numbers, helping doctors
to make a diagnosis.

It is useful to studying neuromuscular function, neuromuscular condition, nerve lesion and reflex response. It
is also important for myoelectric control of prosthetic devices. The EMG signal terminate at the end of the limb
nerve is acquired and used to activate mechanical arm.

Electroretinography (ERG):

Electrical potential exists between cornea and the back of eye. This potential changes when the eye is illuminated.
The process of recording the change in potential when light falls on the eye is called ERG. It can be measured
with a pair of electrodes. One is mounted on the contact lens and is in direct contact with cornea. The other
electrode is placed on the skin adjacent to the other cornea of the eye. Typical signal amplitude is about 500µV.

ERG is an eye test used to detect abnormal function of the retina (the light-detecting portion of the eye).
Specifically, in this test, the light-sensitive cells of the eye, the rods and cones, and their connecting ganglion cells
in the retina are examined.

The measurements given by an ERG are used to confirm or rule out various conditions, including: (applications)

 retinitis pigmentosa, which is a genetic disease causing loss of peripheral and night vision
 macular degeneration, which is a loss of vision due to the death of cells in the macula
 retinoblastoma, which is a cancer of the retina
 retinal separation, which is a detachment of the retina from the back of the eyeball
 cone rod dystrophy (CRD), which is vision loss due to impaired cone and rod cells
An ERG may also help your doctor assess your need for retinal surgery or other types of eye surgery, such as
the removal of cataracts.

Magnetoencephalography (MEG):

MEG scan is imaging technique that identifies the brain activity and measures small magnetic fields produced in
the brain. The scan is used to produce a magnetic source image to pinpoint the source of seizures.

Magnetic fields are detected by extremely sensitive devices called ‘super conducting quantum interface device
(SQUID) and amplifiers.

Page 6 of 7
 Biopotentials Electrodes and Amplifiers , Exam-2018
Department of Applied Physics and Electronic Engineering, University of Rajshahi

A completely noninvasive procedure that uses an array of highly sensitive sensors to detect and record the magnetic
fields associated with electrical activity in the brain. There are many uses for MEG, including determining the
function of various parts of the brain and localizing epileptic activity.

Only MEG can measure fast, millisecond phenomena and also perform localization accurate to the millimeter level.
It does this noninvasively (without injections or radiations of any kind) by measuring the magnetic fields that
naturally emanate whenever electric current flows within the neurons of the brain. The fields being measured are
extremely weak, about billions time smaller than the earth’s magnetic field. The MEG technique uses very
sophisticated instrumentation, sensitive enough to detect these weak signals, while simultaneously discriminating
against interference from the much strong background noise.

Electronystagmography (ENG):

Electronystagmography (ENG) is a test that examines the movement of your eyes in order to assess how well two
cranial nerves within your brain are functioning. The nerves are the acoustic (or vestibulocochlear) nerve (the
nerve connecting the brain and the inner controlling hearing and balance) and the oculomotor nerve (the nerve
connecting the brain to the muscles of the eyes).

Purpose of an ENG:

The ENG is designed to detect disorders in the parts of your inner ear that are responsible for orientation and
balance, as well as in the nerves that connect your brain to your eyes and ears.

This test is recommended if someone is suffering from any of the following symptoms:

1. Dizziness
2. Vertigo
3. Problem with balance
4. hearing loss

Page 7 of 7