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

6.2. Biomeasurement circuit
Figure 36 shows the basic elements which constitute basic bioinstrumentation system. Circuits
play a very important role in bioinstrumentation system. If a bioinstrumentation needs to be
developed or improved to be fit for new condition, function circuits in different blocks from
figure 36 have to be respectively designed to form bioinstrumentation system with relative
indexes. Among all kinds of circuits, amplifiers and A/D converters are very important
component for detecting the biological signal. Hence, amplifying circuits will be only intro‐
duced here in detail, and other function circuits could be read or utilized in relative books.
• Bioelectric amplifier
In order to record the bioelectric potential from the body, biological amplification is always
required. The simplest form is shown in figure 37 which uses a single-input amplifier. Here
amplifier only amplifies one input signal which is applied in the input and the reference
“earth” or “ground”.
In this amplifier, the resistor 
R
1
is required to allow the “bias current” to flow into the non-
inverting (+) input of the operational amplifier and the resistor 
R
2
is required to balance the
resistor 
R
1
so that the bias currents do not produce a voltage difference between the two inputs
of the amplifier. If there is no capacitor in the positive input, amplifier in figure 37 will become
a voltage follower, that’s to say, the bioelectric input signal could be transmitted completely
to the output. Namely, the following equation is given:
u
in+
=u
out
Unfortunately, in circuit shown in figure 37, the resistor 
R
1
defines the maximum input
impedance of the amplifier. The input impedance is an important consideration in bioelectric
amplifiers because it can cause attenuation of a signal which is derived from electrodes with
high impedances. For example, if the two electrode impedances were 10kΩand the input
impedance of the amplifier was 1MΩ, then 1% of the signal would be lost by attenuation of
Biomedical Sensor, Device and Measurement Systems
http://dx.doi.org/10.5772/59941
221

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two electrodes. The impedance presented by electrodes is termed the source impedance which
has to be very much less than the input impedance of amplifier. Source impedance is very
important when we consider differential amplifier shortly.