Download Biomedical Instrumentation Electrodes

University of Zagreb
Faculty of Electrical Engineering and
Computing
Biomedical Instrumentation
Biopotential amplifiers
prof.dr.sc. Ratko Magjarevid
October 2016
Biopotential Amplifier
• Basic function
• To increase the amplitude of weak electric signals of
biological origin
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Biopotential Amplifier
The basic requirements to satisfy :
• no influence to the monitored physiological process
• no distorsion of the measured signal should not be distorted
• separation of signal and interferences/noise
• protection of the patient from any hazard (primarily from
electrical shock)
• the amplifier itself has to be protected against any damages from
electrical power supply (230V/50 Hz) or other medical equipment
(electrosurgical devices, defibrillators)
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Biopotential Amplifier
• The most important part of any equipment recording
bioelectric potentials is the input amplifier
• Most important biopotential amplifier
characteristics:
– Differential measurement (differentia or instrumetation
amplifier)
– High gain (input signal 50uV to 1 mV)
– High common mode rejection ratio (CMRR)
– Frequency range typically from 0,05 Hz to >=100 Hz
– Very high input impedance
– Low noise
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Input Signal
• Composite signal
– useful signal – e.g. ECG: amplitude span from 50 uV
to 1 mV
– polarisation voltage (electrochemical contact
potential @ electrodes) – DC component, up to 300
mV
– interference – mains (50 Hz or 60 Hz), up to 100 mV
– interference voltages – defibrilator shock (n x 1000 V)
or RF surgery equipment voltages
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Symplified Biopotential Amplifier
Block Diagram
Protection circuit
Lead Selector
Preamplifier
Calibration Circuit
Isolation Circuit
Processing
(analog or digital)
Electrodes
Driver Amplifier +
Recorder - Printer
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Heart Dipole
• Electrical activity of the heart is represented by a dipole
• Changes of the dipole magnitude and orientation cause
detectable changes in the electric field
• These changes are measurable at the body surface (A and B)
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Figure 6.2 Relationships between the two lead vectors a1 and a2 and the
cardiac vector M. The component of M in the direction of a1 is given by the
dot product of these two vectors and denoted on the figure by va1. Lead
vector a2 is perpendicular to the cardiac vector, so no voltage component is
seen in this lead.
Differential Amplifier
• A differential amplifier
– amplifies the difference between two input
voltages
– suppresses any voltage common to the two inputs
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Common Mode Rejection Ratio
(CMRR)
•
•
•
•
The ratio of the differential gain over the common-mode gain
Expressed in decibels
Typically 100 – 120 dB for integrated instrumentation amplifiers
Function of frequency and source-impedance unbalance.
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Measurement of CMRR
AD
H
AZ
uizlD
AD 
uulD
Δ
uizlZ
AZ 
uulZ
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Differential and common mode voltages
• The origin of differential voltage is
biopotential
• What is the origin of common mode voltage?
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Electrodes
Electrode is an interface
• to connect the measurement devices and
measure bioelectrical potentials, electrode is
used as an interface, however..
The electrode is also a transducer
• exchange charge carriers :
– in electrical circuits, electrons are charge carriers
– in the body, ions are charge carriers
• connects to the surface of the body (skin, mucous
membranes) or on/in the organ inside the body
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Electrodes
• Most of bioelectric potentials strive to measure
noninvasively, e.g. from the surface of the body, by
placing electrodes on the skin
• Electrical characteristics of different tissues
– specific conductivity (specific resistance)
– specific dielectric constant
• Characteristics of biological tissue are:
– nonlinearity (dependence on frequency and current density),
– inhomogenity (unequal material properties of the body)
– anisotropy (different properties in different dirrections, typically along
the fiber-cells)
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Electrodes
• Using a model of the interface for better understanding
of the interface electrode -tissue
• Passive electrical characteristics of the skin - electrode
interface strive to express by ideal electric components
with intent parameters
– Resistance
– Capacity
• This model can be used for measurement electrodes in
limited frequency range
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Equivalent circuit of the skinelectrode
Electrode
Skin
Virtual electrode
Biological
issue
Electrode – skin intarface and its simplified electrical
circuit
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Metal-electrolyte potential
Standard electrode potential relative to standard hydrogen electrode at 20°C
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Polarization voltage
• If these two solutions are separated with semi-permeable
membrane to allow passage of ions, and to avoid the
original combination of solutions, the potential difference between
the solutions can be measured according to the formula
E  E0.5 M 1  E0 M 2
RT [cM 1 ]
 E0 M 1  E0 M 2 
ln
nF [cM 2 ]
• Each electrode that comes in contact with the electrolyte will
have the potential of the expression above. This potential
is undesirable in the measurement of biological voltage
because when using high gain dc amplifier, it causes saturation
of the amplifier. To avoid saturation, amplifier with less gain in the
input is used and the next stages of amplification are
separated with condenser.
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Equivalent circuit of the skinelectrode
• Measurement circuit
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Input Impedance of the Amplifier
• Differential amplifier
– Differential amplification
𝐴𝑑𝑖𝑓𝑓 = 𝑅6 /𝑅3
R3 =R4
R5 =R6
– Input differential impedance
𝑍 𝑑𝑖𝑓𝑓 = 𝑅3 + 𝑅4
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Input Impedance Disballance
• Differential amplifier + skin
• Skin impedance causes disballance in + and – branches of the
amplifier input
• What is the amplification of the amplifier for the input
differertial signal?
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Input Impedance Disballance
𝐴= 1+
𝑅6
𝑅3 +𝑍2
𝑅5
)u
𝑅4 +𝑍1 +
(
-
𝑅6
𝑅3 +𝑍2
u23
Input Impedance Disballance
• Differential amplifier + skin + biopotential source
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EM Interference
From:
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Instrumentation Amplifier
• a type of differential amplifier that has been outfitted with input
buffer amplifiers
• eliminates the need for input impedance matching
• particularly suitable for use in measurement of bioelectric
potentials
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Amplifier Circuits - DC coupled
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Amplifier Circuits - AC coupled
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Amplifier Circuits - AC coupled
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Figure 6.18 This ECG amplifier has a gain of 25 in the dc-coupled stages. The
high-pass filter feeds a noninverting-amplifier stage that has a gain of 32. The
total gain is 25 X 32 = 800. When mA 776 op amps were used, the circuit was
found to have a CMRR of 86 dB at 100 Hz and a noise level of 40 mV peak to
peak at the output. The frequency response was 0.04 to 150 Hz for ±3 dB and
was flat over 4 to 40 Hz. A single op amp chip, the LM 324, that contains four
individual op amps could also be used in this circuit reducing the total parts
count.
Amplifier Circuits - AC coupled
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Auto-zero amplifiers
• Automatic nihilation of amplifier offset
voltage
u  u  u
u  A2  uizl  A2  A1  u
uizl  A1  u
u  u  A2 A1  u
u  u 1  A1 A2 
u 
u 
A u
Au
u
 1 off  1 off
1  A1 A2 1  A1 A2
A1 A2
uoff
A2
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Right Leg Drive Circuitry
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Isolation amplifiers
• Design of isolation amps:
– Optical coupling
•
•
•
•
•
Isolation voltage typ. 4 – 7 kV
fast
cheap
Nonlinear – digitazing of signals before the isolation gap
Noice high
– Electromagnetic coupling
• Isolation voltage up to 10 kV
• Resolution typ. 12 bit, max. 16 bit
• fg low, max 1 kHz
– Capacitive coupling
• Characteristics worse than other types, but cheapest
Isolation amplifiers
• Galvanic isolation of sensory and the measurement part of
the measurement system (attached to the patient) and the
processing and display part, usually powered by mains
• Floating principle of measurement of biopotentials
• The aim is also to protect the patient from the potentially
dangerous voltages or currents comming from the un-isolated
(mains powered part) of the system
Biopotential Isolation Amplifier
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Principle of floating measurements
lim H 
Ad

u2i
u1i
za Z n  
Otically coupled isolation amp
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Linearisation
u2  f
1
 u1 
 
 R1 
u3
 f '  u2 
R3
i2  f  u1 
i2 '  f '  u3 
 1  u1  
u3
 f '  f  
R3
 R1  

 1  u1   u1
u3
 f '  f   
R3
 R1   R1

u3 R3
f  u1   f '  u3 

u1 R1
u1  u3
Linearisation
EM coupled amps - AD 215
Capacitavly coupled isolation amps
 ISO
124
 The input signal is frequency modulated
 fosc 500 kHz; Vizo = 2,4 kVef
Capacitively coupled isolation amps
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Digital isolation amp principles
Protection of the input of the amplifier
•Diodes
•Zener diodes
•Gas-discharge tubes
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Input guarding
• Increases:
– input impedance of the amplifier
– CMRR
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Literature
• John G. Webster: Medical Instrumentation,
Chapter 6, Biopotential Amplifiers
• Homework: Problems 5.17 and 5.18
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