Selection of our books indexed in the Book Citation Index in Web of Science™ Core Collection (bkci)

5. Signal filters and operational amplifiers
Biosignals are recorded as potentials, voltages, and electrical field strengths generated by
nerves and muscles. The measurements involve voltages at very low levels, typically ranging
from 1μV to 100mV, with high source impedances and superimposed high level interference
signals and noise. The signals need to be amplified to make them compatible with devices such
displays, recorders, or A/D converters for computerized equipment. Amplifiers adequately to
measure these signals have to satisfy very specific requirements. They have to provide
amplification selective to the physiological signal, reject superimposed noise and interference
signals, and guarantee protection from damages through voltage and current surges for both
patient and electronic equipment. Amplifiers featuring these specifications are known as
biopotential amplifiers.
5.1. Basic signal amplifier
The basic requirements that a biopotential amplifier has to satisfy are:
1. The physiological process to be monitored should not be influenced in any way by the
amplifier.
2. The measurement signals should not be distorted.
3. The amplifier has to provide protection of patient from any hazard of electrical shock.
4. The amplifier itself has to be protected against damages that might result from high input
voltages as they occur during the application of defibrillators or electrosurgical instru‐
mentation.
A typical configuration for the measurement of biopotentials as illustrated in figure 29. Three
electrodes, two of them are used to pick up the biological signal and the third providing the
reference potential, connect the subject to amplifier. The input signal to the amplifier consists
of five components:(1) the desired biopotential, (2)undesired biopotential, (3) a power line
interference signal of 60Hz (50Hz in some countries) and its harmonics, (4)interference signal
generated by the tissue/electrode interface, and (5) noise. Proper design of the amplifier
provides rejection of a large portion of the signal interferences. The main task of designing
deferential amplifier is to reject the line frequency interference that is electrostatically or
magnetically coupled into subject. The desired biopotential appears as a voltage between two
input terminals of differential amplifier and is referred to as the differential signal. The line
frequency reference signal shows only small differences in amplitude and phase between the
two measuring electrodes, causing approximately the same potential at both inputs, and thus
Advances in Bioengineering
212

---

appears only between the inputs and ground and is called common mode signal. Strong
rejection of the common mode signal is one of the most important characteristics of a good
biopotential amplifier.
In order to provide optimum signal quality and adequate voltage level for further signal
processing, amplifier has to provide a suitable gain range and needs to maintain a possible
signal-to-noise ratio. The presence of the high level interference signals not only deteriorates
the quality of the physiological signals, but also restricts the design of the biopotential
amplifier. For example, electrode half-cell biopotentials limit the gain factor of the first
amplifier stage since their amplitude can be several orders of magnitude larger than the
amplitude of physiological signal. To prevent the amplifier from going to saturation, this
component has to be eliminated before the required gain be provided for physiological signal.