Biomedical Engineering Reference
In-Depth Information
better. Note that the diodes have been reversed to obtain a positive rectified signal. The
second stage inverts the signal polarity. To obtain improved high-frequency response, the
resistor values should be reduced.
This circuit is sensitive to source impedance, so it is important to ensure that it is driven
from a low impedance, such as an op amp buffer stage. The circuit has good linearity down
to a couple of mV at low frequencies but has a limited high-frequency response. Use of
high-speed diodes, lower resistance values, and faster op amps is recommended if greater
sensitivity or higher-frequency response is required.
The output of the rectifier is passed through a low-pass filter to produce the envelope.
5.4.3.5 Myoelectric Signal Processing
Developing a differential amplifier for EMG poses few real problems if the appropriate
precautions are taken. When a muscle is caused to contract, the distribution of electrolytes
within the tissue changes, which induces small voltages on the surface of the skin that
can be picked up using the appropriate electrodes. From the specifications in Table 5-1,
the voltage levels vary from 50
V up to a maximum of 5 mV. However, this signal
will probably be contaminated by other much larger biopotential signals and certainly by
induced
mains hum.
The problem is to sense and isolate this signal so that it can be used to control the
movement of a prosthetic device by driving a pneumatic artificial muscle (PAM), discussed
in Chapter 3.
The first requirement is to select an appropriate set of electrodes to detect this small
voltage with the addition of as few measurement artifacts as possible. Electrode types
were discussed in Chapter 2, and from this discussion it is obvious that a silver-silver
chloride surface electrode and a conductive gel are best. These electrodes can be applied
directly over the muscle complex of interest. The electrode interface is reasonably high
impedance and also picks up 50 Hz mains, ECG signals, and myriad other electrical noise
generated by the body and from extraneous sources.
An instrumentation amplifier is required to provide a high input impedance, a good
CMRR, and the appropriate gain to amplify the small signal to a usable level for further
processing. This amplifier must be provided with a pair of differential inputs to sense the
myoelectric signal and a ground reference, as shown in Figure 5-35.
This instrumentation configuration can easily achieve a CMRR in excess of 60 dB to
eliminate most of the common-mode 50 Hz while still providing a voltage gain of 100.
This will provide an output of between 5 mV and 500 mV.
The next problem to be solved is that of electrode movement. As discussed in Chap-
ter 2, electrode movement alters the cell potential of the conductive gel, which results in
the generation of large electrical signals called motion artifacts. These can saturate any
further stages of amplification and need to be removed.
Fortunately, motion artifacts are found at the lower end of the spectral response of
the EMG signal and can therefore be removed by high-pass filtering without removing
too much of the real signal. A first-order high-pass filter witha5Hzcutoff frequency
realized by
C
1
and
R
1
, followed by an amplifier with a voltage gain of 10, can easily
be designed to implement this function, as shown in Figure 5-36. Note that a small ca-
pacitor,
C
2
, is included in the feedback path to reduce the high-frequency gain of the
amplifier. This eliminates high-frequency noise above the maximum bandwidth of the
EMG signal.
μ
