Biomedical Engineering Reference
In-Depth Information
FIGURE 5-72
Digital opto-isolator
schematic.
FIGURE 5-73
Opto-isolated
analog link.
the current flow through the phototransistor in an identical manner. Note that in this con-
figuration the digital signal is inverted.
Galvanic isolation of analog signals can be achieved in a number of ways. The simplest
is to convert the analog signal to a digital one, using pulse-width modulation, voltage-to-
frequency conversion, or even an ADC to generate a digital word. This digital information
is then transmitted across the barrier using the method shown in Figure 5-72 before
conversion back to an analog signal.
For pulse-width modulation, the conversion is achieved using a simple low-pass filter,
whereas in the frequency-encoded signal a frequency-to-voltage converter is used. In the
case where a digital word is transmitted, a conventional DAC converts the word back to
its analog equivalent
An interesting alternative is a true analog link, where the intensity of the LED encodes
for the analog voltage, as shown in Figure 5-73. In this configuration, the isolation consists
of a pair of matched photodiodes and an LED. The signal from the photodiode is fed back
to the input of the amplifier to counteract the nonlinear characteristics of both the LED
and the photodiodes.
5.8
REFERENCES
Alciatore, D. and M. Histand. (2003).
Introduction to Mechatronics and Measurement Systems,
2d
ed. Boston: McGraw Hill.
Benedict, T. and G. Bordner. (1962). “Synthesis of an Optimal Set of Radar Track While Scan
Smoothing Equations.”
IRE Transactions on Automatic Control
AC-7
: 27-32.
