Biomedical Engineering Reference
In-Depth Information
The current flow through the electrode resistances will cause a voltage drop
so that the voltage at the input terminals
V
in
will be less than the desired
signal
V
EMG
. For example, if
R
s1
=
80,000
,a
2-mV EMG signal will be reduced to 1.6 mV at
V
in. A voltage loss of
0.2 mV occurs across each of the electrodes. If
R
s1
and
R
s2
were decreased
by better skin preparation to 1000
, and
R
i
were increased to 1 M
,the
2-mV EMG signal would be reduced only slightly, to 1.998 mV. Thus, it is
desirable to have input impedances of 1 M
or higher, and to prepare the
skin to reduce the impedance to 1000
or less. For indwelling electrodes,
the electrode impedance can be as high as 50,000
, so an amplifier with at
least 5 M
input impedance should be used.
R
s2
=
10,000
and
R
i
=
10.2.3 Frequency Response
The frequency bandwidth of an EMG amplifier should be such as to amplify,
without attenuation, all frequencies present in the EMG. The bandwidth of
any amplifier, as shown in Figure 10.6, is the difference between the upper
cutoff frequency
f
2
and the lower cutoff frequency
f
1
. The gain of the amplifier
at these cutoff frequencies is 0.707 of the gain in the midfrequency region. If
we express the midfrequency gain as 100%, the gain at the cutoff frequencies
has dropped to 70.7%, or the power has dropped to
(
0
.
707
)
2
0
.
5. These are
also referred to as the half-power points. Often amplifier gain is expressed in
logarithmic form and expressed in decibels,
=
gain
(
dB
)
=
20 log
10
(
linear gain
)
(10.3)
If the linear gain were 1000, the gain in decibels would be 60, and the gain
at the cutoff frequency would be 57 dB (3 dB less than that at midfrequency).
In a high-fidelity amplifier used for music reproduction,
f
1
and
f
2
are
designed to accommodate the range of human hearing, from 50 to 20,000 Hz.
All frequencies present in the music will be amplified equally, producing a
Figure 10.6
Frequency response of the biological amplifier showing a gain of 1000
(60 dB), and lower and upper cutoff frequencies
f
1
and
f
2
.
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