SIGNAL SOURCES FOR SIMULATION, TESTING, AND CALIBRATION - Design and Development of Medical Electronic Instrumentation - page 250

Figure 6.1 This module generates an extremely accurate, low-amplitude (10 µ V p-p to 1 mV p-p ) sine-

wave signal for precise calibration of biopotential ampli ers. Because of its high stability (sine-wave

distortion of less than 0.2%), this circuit is ideal for measuring channel phase shift in topographic
brain mappers and other biopotential array ampli

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fi ers.

power output stage to decouple the precision quadrature oscillator from the output load. R5
is used to trim the amplitude of the signal at the op-amp output to exactly 1 V p-p . R7 and R8
form an attenuator that divides the signal at the output of the op-amp to any amplitude
desired between 10 µ V p-p and 1 mV p-p . To calibrate a single-ended biopotential ampli fi er,

connect the ACTIVE input of the ampli fi er to J1-1 and the REFERENCE input to J1-2. For

di ff erential ampli fi ers, connect both REFERENCE and SUBJECT GROUND to J1-2.

ANALOG GENERATION OF ARBITRARY WAVEFORMS

In many applications, repetitive sine, square, and triangle waves are seldom representative
of the signals that the equipment under test is designed to process. For example, the heart's
electrical signal is a waveform consisting of a complex mixture of these basic waveshapes
intertwined with intermittent baseline segments. Since a constant live feed of such signals
may be impractical or even dangerous for testing biomedical equipment, dedicated signal
sources had to be developed to be capable of synthesizing waveforms similar to those gen-
erated by their physiological counterparts. Similar requirements are evident for the gener-
ation of test signals representative of those produced by medical imaging sensors and other
sources that cannot be simulated by plain sines, ramps, or square waves.
Figure 6.3 shows a simple circuit that can generate multiple synchronized repetitive
waveforms of arbitrary shape. IC2 is a binary counter that causes the outputs of IC3 to go
high in sequence. One at a time, each of these lines causes current to ow through R21 by

way of its associated diode (D1-D15) and linear slider potentiometers (R1-R15). As the
counter cycles, a stepped waveform appears across R21.
Think about a graphic equalizer—your stereo set probably has one, or you have seen
them in pictures of recording studios. This type of equalizer is a multiband variable audio
fi

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filter using slide controls as the amplitude-adjustable elements. It's named graphic for the
positions of the sliders graphing the resulting frequency response of the equalizer. In a sim-
ilar way, the basic shape of the stepped waveform across R21 tracks the shape described
by the linear slider potentiometers as shown in Figure 6.4. Counter IC2 is clocked by the