Dept. of Biomed. Eng. BME302: Biomedical Instrumentation Kyung Hee Univ.
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Chapter 6 Biopotential Amplifiers
6.1 Basic Requirements
Use of biopotential amplifiers
Increase the amplitude (voltage, current, power) of biopotential signals
Isolation of load from source
Basic requirements of biopotential amplifiers
High input impedance for minimal loading: > 10 M
Input protection
Isolation for safety
Low output impedance
Optimal bandwidth for better SNR
Enough gain: ~ 1000 or more
High CMRR for differential input amplifiers
Quick calibration
6.2 The Electrocardiography
The ECG
Dipole model of electrical activity of the heart
The dipole changes its magnitude and orientation during cardiac cycle
Dipole moment is called the cardiac vector
Volume conductor problem produces time-varying equipotential lines on the body
surface
A pair of electrodes on the surface or its equivalence defines a lead
Projection of the cardiac vector to a lead vector is scalar voltage of the lead
Frontal plane ECG
Electrodes: RA, LA, LL, and RL (ground)
Bipolar leads
Lead I: LA(+) RA(), lead II: LL(+) RA(), lead III: LL(+) LA()
Eindhoven’s triangle and Kirchhoff’s voltage law I II + III = 0 Wilson’s central terminal (W), v v v vW RA LA LL
Unipolar leads: small amplitude
Dept. of Biomed. Eng. BME302: Biomedical Instrumentation Kyung Hee Univ.
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Lead VR: RA(+) W(), lead VL: LA(+) W(), lead VF: LL(+) W()
Augmented unipolar leads: 50 % larger amplitude than unipolar leads
Lead aVR: RA’(+) W(), lead aVL: LA’(+) W(), lead aVF: LL’(+)
W()
Transverse plane ECG
Precordial (chest) leads
Lead V1, V2, V3, V4, V5, and V6
Lead Vi: Vi(+) W()
Posterior leads: esophageal electrode (+) W()
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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 val. Lead vector a2 is perpendicular to the cardiac vector, so no voltage component is seen in this lead.
a1
a2
a1
M
+
Dept. of Biomed. Eng. BME302: Biomedical Instrumentation Kyung Hee Univ.
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Dept. of Biomed. Eng. BME302: Biomedical Instrumentation Kyung Hee Univ.
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Specific Requirements of the Electrocardiograph
AAMI/ANSI
FDA
Functional Blocks of the Electrocardiograph
Protection circuit: protection of the amplifier from large transient shock
Lead selector: resistor network or software
Calibration signal: 1-mV calibration pulse
Preamplifier: instrumentation amplifier, high CMRR, gain control
Isolation circuit: safety of the patient, separate patient ground from earth ground
Driven-right-leg circuit
Driver amplifier: ac coupling, bandpass filter, zero-offset control
Memory system
Microcomputer: control, digital signal processing, storage, user interface,
communication, etc.
Recorder or printer: hard copy, thermal or electrostatic recording
Dept. of Biomed. Eng. BME302: Biomedical Instrumentation Kyung Hee Univ.
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6.3 Problems Frequently Encountered
Frequency Distortion
Cause: inappropriate design
Desirable bandwidth: 0.05 – 150 Hz in most diagnostic ECG
High-frequency distortion: sharp corners are smoothed, QRS complex is reduced
Low-frequency distortion: baseline distortion, monophasic wave becomes biphasic
Saturation or Cutoff Distortion
Cause: high offset voltage or maladjustments of amplifier
Higher or lower portions of ECG are cut off
Ground Loops
Cause: multiple instruments with different ground potentials on one patient
May cause safety problem
Increased common mode voltage
Figure 6.7 Block diagram of an electrocardiograph
Right legelectrode
Controlprogram
Microcomputer
ECG analysisprogram
Operatordisplay
Keyboard
Drivenright legcircuit
Amplifierprotectioncircuit
Leadselector
Sensingelectrodes
Lead-faildetect
Preamplifier
Autocalibration
Baselinerestoration
Isolatedpowersupply
Isolationcircuit
Driveramplifier
RecorderÐprinter
ADC Memory
Parallel circuits for simultaneous recordings from different leads
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Open Lead Wires
Cause: lead wire breakage, electrode falls off, poor electrode-skin contact
Excessive power-line noise, loss of signal
Artifact from Large Electric Transients
Cause: defibrillator, ESU, large motion artifact, static electric discharge, lead
switching, etc.
High-amplitude pulse or step amplifier saturation slow recovery by the time
constant of lower cutoff frequency
Protection circuit usually speeds up the recovery
Interference from Electric Devices
Power line interference: electric field coupling and magnetic field coupling
Electric field coupling between power lines and electrocardiograph
id 3 has no effect due to chassis ground
v v Z Z i Z Z i i Z Zd d dA B G G 1 1 2 2 1 1 2b g b g b g : differential mode input
noise
For 9 m cables, i id d1 2 6 nA . If Z Z1 2 20 b g k , v vA B = 120 V.
To reduce this interference,
Shield the leads and ground each shield at the electrocardiograph
Lower skin-electrode impedance.
Electric field coupling between power lines and patient (capacitive coupling)
v i Zcm db G : common mode voltage
Typically, vcm A k mV 0 2 50 10. b g b g . In poor electrical environments,
Figure 6.8 Effect of a voltage transient on an ECG recorded on an electrocardiograph in which the transient causes the amplifier to saturate, and a finite period of time is required for the charge to bleed off enough to bring the ECG back into the amplifier’s active region of operation. This is followed by a first-order recovery of the system.
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idb > 1 A and vcm > 50 mV is possible.
v v vZ
Z Z
Z
Z Zv
Z Z
ZA B cmin
in 1
in
in 2cm
2 1
in
FHG
IKJ
FHG
IKJ : differential mode input
noise
Typically, v vA B mV k
5 M FHG
IKJ 10
2040b g
V.
To reduce this interference,
Lower skin-electrode impedance
Reduce the impedance imbalance, Z Z2 1b g Increase amplifier input impedance.
Magnetic field coupling (magnetic induction)
Coupling coil: single turn, electrocardiograph-lead wires-patient
Interference voltage magnetic field strength and area of the coil
To reduce this interference,
Reduce the magnetic field by magnetic shielding
Keep the electrocardiograph away from magnetic field
Reduce the effective area of the coil by twisting lead wires.
Other Sources of Electric Interference
Electromagnetic interferences
Nearby radio, television, radar facilities
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Lead wires and patient serve as an antenna
The p-n junctions of semiconductor devices or electrode-electrolyte interface
can rectify RF signals
High-frequency generators in the hospital
Electrosurgical and diathermy equipment
X-ray machines
Switches and relays of heavy-duty electric equipment
Arcing in a fluorescent light that is flickering
Electromyography signal from muscles
Figure 6.10 A mechanism of electric-field pickup of an electrocardiograph resulting from the power line. Coupling capacitance between the hot side of the power line and lead wires causes current to flow through skin-electrode impedances on its way to ground.
Electrocardiograph
A
Power line 120 V
B
G
C3C1
Z1
Z2
ZG
C2
Id1
Id2
Id1+ Id2
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Figure 6.11 Current flows from the power line through the body and ground impedance, thus creating a common-mode voltage everywhere on the body. Zin is not only resistive but, as a result of RF bypass capacitors at the amplifier input, has a reactive component as well.
Electrocardiograph
Power line 120 V
A
Zin
Z1
Cb
idb
ZG
Z2
cm
B
G
Zin
cm
cm
idb
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6.4 Transient Protection
Protection of amplifier from defibrillator, electrosurgical unit, static discharge, etc.
Voltage limiting device
Low-voltage breakdown: parallel silicon diode, about 600 mV per pair, may
distort ECG signal
Moderate-voltage breakdown: back-to-back Zener diodes, 2 – 20 V
High-voltage breakdown: gas-discharge tube (miniature neon lamp), 50 - 90
V
High power series resistor is used to limit the amplifier’s input current.
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6.5 Common-Mode and Other Interference-Reduction Circuits
Electric- and Magnetic-Field Pickup
Shielding
Grounded, continuous solid metal
Grounded, copper screen
Magnetic shielding with high permeability material such sheet steel
Driven-Right-Leg System
Common model voltage reduction by negative feedback
Improve electric safety
Figure 6.15 Driven-right-leg circuit for minimizing common- mode interference The circuit derives common-mode voltage from a pair of averaging resistors connected to v3 and v4in Figure 3.5. The right leg is not grounded but is connected to output of the auxiliary op amp.
id
Ra
RRL
Ra
Rf
RoAuxiliaryop amp
+
+
+
RL
4
cm
3
Dept. of Biomed. Eng. BME302: Biomedical Instrumentation Kyung Hee Univ.
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6.6 Amplifiers for Other Biopotential Signals
Biopotential
Different amplitude: tens of V – 100 mV
Different bandwidth requirement: dc – 10 kHz
Different electrodes different amplifier input circuits
EMG Amplifier
Amplitude: 100 V – 90 mV
Skin surface electrodes: 0.1 – 1 mV amplitude, 200 – 5000 electrode
impedance
Intramuscular needle electrodes: higher amplitude, higher electrode impedance
Frequency: 25 Hz – several kHz
Amplifiers for Use with Glass Micropipet Intracellular Electrodes
Microelectrodes or intracellular electrodes
Very high electrode impedance
Large shunting capacitance
Bandwidth requirements
RMP: dc, 50 – 100 mV
Action potential: up to 10 kHz
Figure E6.1 Equivalent circuit of driven-right-leg system of Figure 6.19.
+
+
RRL
id
Ro
Rf
id
o/Rf2cm/Ra
Ra/2
o
cm
cm
Dept. of Biomed. Eng. BME302: Biomedical Instrumentation Kyung Hee Univ.
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Negative input capacitance amplifier: low-gain, very-high-input-impedance,
noninverting amplifier with positive feedback, may be unstable and noisy
EEG Amplifiers
Amplitude: 25 – 100 V using smaller surface electrodes
Somewhat higher electrode impedance than ECG
Frequency: 0.1 – 100 Hz
Higher CMRR and low noise amplifier
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6.7 Example of Biopotential Preamplifier
Instrumentation amplifier + highpass filter + noninverting amplifier with gain and
lowpass cutoff frequency
Low noise
Small input current
High input impedance
No saturation from dc offset voltage
High CMRR
Optimal bandwidth
Enough gain
Fast recovery from saturation
Figure 6.17 (a) Basic arrangement for negative-input capacitance amplifier. Basic amplifier is on the right-hand side; equivalent source with lumped series resistance Rs and shunt capacitance Cs is on the left. (b) Equivalent circuit of basic negative-input capacitance amplifier.
Rs
o
vi viAvo
Av
Cf
ii Cf
+
+
++
+
+
(b)(a)
CfEs
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6.8 Other Biopotential Signal Processors
Cardiotachometers
Determine HR from ECG, arterial blood pressure, etc.
Two types of cardiotachometer
Averaging cardiotachometer
Beat-to-beat cardiotachometer
Alarm circuits
QRS detection by DSP is most often used with electrocardiograph and patient
monitor
Electromyogram Integrators
Quantification of the amount of EMG activity
Full-wave rectification and integration by hardware or DSP
Dept. of Biomed. Eng. BME302: Biomedical Instrumentation Kyung Hee Univ.
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Evoked Potentials and Signal Averaging
Evoked potential: neurological response to a particular stimulus
Signal averaging
Signal + random noise
Repeated measurements
Means for alignment of each recording
SNR is improved by a factor of n from n times of averaging
Figure 6.21 Block diagram of an integrator for EMG signals
Switch
Absolute-valuecircuit
Monostablemultivibrator
Comparator
CEMG
Integrator
+
R
P1
v t
1
Counter
2
3
Dept. of Biomed. Eng. BME302: Biomedical Instrumentation Kyung Hee Univ.
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Sensory nerve response due to electric stimulus
EEG due to somatosensory (electric), visual (light), or auditory (sound) stimulus
Fetal ECG and HRECG with R-wave as a reference
Fetal Electrocardiography
Amplitude of fetal ECG: less than 50 V
Electrodes at the abdomen of the mother
Stronger ECG of the mother
EMG of the mother
Motion artifact
Signal averaging
Anticoincidence detectors
Adaptive filtering technique
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Vectorcardiography
Vectorcardiogram (VCG): three-dimensional or two-dimensional picture of the
orientation and magnitude of the cardiac vector through one cardiac cycle
XYZ lead
X-Y plot using X and Y lead with intensity modulation using Z lead
6.9 Cardiac Monitor or Cardioscope
Continuous monitoring of ECG and HR
Surgery and anesthesia
Recovering MI patients
During labor, etc.
Cardiac monitor or cardioscope
ECG electrodes
Protection circuit
Lead fault detector
Ac type: impedance measurement using 50 kHz, 100 – 200 A current
Dc type: impedance measurement using dc, < 1 A current
Pacemaker pulse rejection circuit
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ECG preamplifier with lower bandwidth (0.67 – 40 Hz)
Isolation circuit
Cardiotachometer
Monitor and recorder
Alarm
Patient monitoring system in ICU, CCU, OR, etc.
Figure 6.26 Block diagram of a cardiac monitor.
Patient
Bus
Electrodes Preamplifier
RAM Displayscreen
Isolation Amplifier
ProgramPROM
Chartrecorder
Storagemedium
Keyboard Alarmindicator
Analog todigitalconverter
MicrocomputerCPU
Communicationport
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6.10 Biotelemetry
Radiotelemetry
Battery operated transmitter completely isolates patients from ground
Modulation: FM, PWM, PPM, PCM, etc.
Multiplexing
Frequency division multiplexing (FDM)
Time division multiplexing (TDM)
Noise, antenna orientation, reliability, etc.
Infrared telemetry
Reliable within short distance
Near infrared signal as carrier
Dept. of Biomed. Eng. BME302: Biomedical Instrumentation Kyung Hee Univ.
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