Biomedical Engineering Reference
In-Depth Information
Figure 3.10 In this analog signal isolator, the analog signal is converted into a pulse train of vari-
able pulse width, which is then used to drive the optoisolator. At the other side of the optoisolator,
the pulse train is demodulated to render the original signal. The large cylinder at the center of the
board is an Optek OPI1264 optoisolator through which isolation levels of 10 kV can be obtained.
A similar circuit can be built using an integrated voltage-to-frequency converter. The
circuit of Figure 3.12 is an application suggested by Analog Devices for its ADVFC32
integrated circuit. This chip is an industry-standard, low-cost monolithic voltage-to-fre-
quency (V/F) converter or frequency-to-voltage (F/V) converter with good linearity and
operating frequency up to 500 kHz. In the V/F con
guration, positive or negative input
voltages or currents can be converted to a proportional frequency using only a few exter-
nal components. For F/V conversion, the same components are used with a simple biasing
network to accommodate a wide range of input logic levels.
In an analog isolator circuit, an input signal in the range 0 to 10 V drives IC1, an
ADVFC32 con
fi
set resistor R2 have
been selected such that a 0-V input causes IC1 to oscillate at 50 kHz, while a 10-V input
yields an output of 500 kHz. IC2 is a bandgap voltage regulator used to generate the of
fi
gured as a V/F converter. Input resistor R1 and o
ff
set
voltage reference. IC3, a high-frequency optoisolator with a high isolation voltage rating,
is used to transmit the frequency-modulated signal generated by IC1 across the isolation
barrier to IC4, an ADVFC32 con
ff
gured as a F/V converter. Integration for the V/F func-
tion is provided by an internal op-amp that has C12 within its feedback loop. This capac-
itor de
fi
fi
nes the frequency response of the isolation ampli
fi
er circuit. With 1000 pF, the
bandwidth of the ampli
set to reproduce the
input range 0 to 10 V by summing a current produced through R9 and R10 by a second
bandgap reference IC5. Prior to exiting the circuit, any remaining carrier is
fi
er is dc to 3 kHz. The output of the V/F is of
ff
fi
filtered via R6
filter.
In the new generation of analog optocouplers that have appeared on the market, the LED
they use has widely variable electrical-to-optical transfer characteristics, just as in any other
optocoupler. However, these optocouplers have a second photodetector, which is used as
part of a feedback loop to stabilize the LED's optical output. The circuit of Figure 3.13 is
very similar to the simple optical analog isolator of Figure 3.8. The main di
and C13, which form a 3-kHz low-pass
fi
erence is that
IC1's output is not simply a current proportional to its input voltage. Rather, part of the
LED's optical
ff
flux is detected by the second photodiode and used to provide feedback to
the op-amp current source. Since the stability of a photodiode is not usually a concern, and
since the characteristics of both photodiodes are closely matched during manufacture, the
fl
Search WWH ::




Custom Search