Basic principles of design of an ECG amplifier - sensitivity [potentials]; frequency response; common mode rejection ratio
1.
Amplifier
a)
Biomedical
parameters are difficult to measure due to their relatively low energies and
similarity of signals from organ to organ. This similarity makes discrimination
of a particular signal difficult. Not only does the transducer have a certain
frequency response and sensitivity to detect a particular signal accurately,
but the signal generated by the transducer requires processing (eg, impedance
matching or filtering of noise) and amplification.
b)
Monitors
require several volts of signal to operate, therefore an amplifier is necessary
to boost the size of the biological signal 1000’s times before display (eg R
wave usually < 2 mv).
2.
Frequency response
a)
Amplifier
for a particular biological signal must be able to respond to the range of
frequencies concerned
b)
Interference
in the signal can be minimised if the bandwidth of the equipment used is
selected to match the signal correctly
c)
Frequency
content of the signal & the frequency response of the monitor are important
determinants of the accuracy of the signal representation and interpretation
d)
Range of
frequencies:
i)
EEG: 1—60
Hz
ii)
ECG:
0.5—100 Hz
iii)
Notice both
include potential for 50 Hz power line interference
iv)
AHA
recommend frequency response of 0.05—100 Hz for ECG analysis
e)
Changes in
the ECG waveform that occur:
i)
Rapidly
(QRS) contain higher frequency information
ii)
More slowly
(ST segment) contain lower frequency information
3.
Common mode rejection ratio
a)
Ability of
an differential amplifier to ignore the interference common to both electrodes
b)
Biological
signals
i)
Very small
signal
ii)
Requires a
very high CMRR
a)
> 1000:1
iii)
A high CMRR
means that the amplifier strongly attenuates signals that are common to both
inputs, but amplifies the difference between the two signals
4.
Sensitivities
a)
The
amplifier must be suitable for the voltage range of the signal
b)
A very
sensitive amplifier is needed for EEG measurement as the size of the signal is
so small
c)
Monitors
require several volts of signal to operate, therefore an amplifier is necessary
to boost the size of the biological signal 1000’s times before display (eg R
wave usually < 2 mv)
d)
The ratio
of the voltage at the output of an amplifier to the signal voltage at the input
is known as the gain of the amplifier
e)
ECG: 0.1 —
6 mv
f)
EEG: 1m — 0.3 mv