Chapter 6 Biopotential Amplifiers - IIRC

Dept. of Biomed. Eng. BME302: Biomedical Instrumentation Kyung Hee Univ.

http://ejwoo.com 1 Eung Je Woo

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



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()



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

+



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



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



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.



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.


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


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



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



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



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.



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



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



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



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



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



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



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



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



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



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

Chapter 6 Biopotential Amplifiers - IIRC

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