HCNR200 Datasheet, PDF (14/16 Pages) Agilent(Hewlett-Packard) – High-Linearity Analog Optocouplers

optimize circuit performance.
Example application circuits will
be discussed later in the data
sheet.
Circuit Design Flexibility
Circuit design with the HCNR200/
201 is very flexible because the
LED and both photodiodes are
accessible to the designer. This
allows the designer to make perf-
ormance trade-offs that would
otherwise be difficult to make with
commercially available isolation
amplifiers (e.g., bandwidth vs.
accuracy vs. cost). Analog isola-
tion circuits can be designed for
applications that have either
unipolar (e.g., 0-10 V) or bipolar
(e.g., ± 10 V) signals, with
positive or negative input or
output voltages. Several simplified
circuit topologies illustrating the
design flexibility of the HCNR200/
201 are discussed below.
The circuit in Figure 12a is
configured to be non-inverting
with positive input and output
voltages. By simply changing the
polarity of one or both of the
photodiodes, the LED, or the op-
amp inputs, it is possible to
implement other circuit configu-
rations as well. Figure 13
illustrates how to change the
basic circuit to accommodate
both positive and negative input
and output voltages. The input
and output circuits can be
matched to achieve any combina-
tion of positive and negative
voltages, allowing for both
inverting and non-inverting
circuits.
All of the configurations described
above are unipolar (single polar-
ity); the circuits cannot accommo-
date a signal that might swing
both positive and negative. It is
possible, however, to use the
HCNR200/201 optocoupler to
implement a bipolar isolation
amplifier. Two topologies that
allow for bipolar operation are
shown in Figure 14.
The circuit in Figure 14a uses two
current sources to offset the
signal so that it appears to be
unipolar to the optocoupler.
Current source I_OS1 provides
enough offset to ensure that I_PD1
is always positive. The second
current source, I_OS2, provides an
offset of opposite polarity to
obtain a net circuit offset of zero.
Current sources I_OS1 and I_OS2 can
be implemented simply as
resistors connected to suitable
voltage sources.
The circuit in Figure 14b uses two
optocouplers to obtain bipolar
operation. The first optocoupler
handles the positive voltage
excursions, while the second
optocoupler handles the negative
ones. The output photodiodes are
connected in an antiparallel
configuration so that they
produce output signals of
opposite polarity.
The first circuit has the obvious
advantage of requiring only one
optocoupler; however, the offset
performance of the circuit is
dependent on the matching of I_OS1
and I_OS2 and is also dependent on
the gain of the optocoupler.
Changes in the gain of the opto-
coupler will directly affect the
offset of the circuit.
The offset performance of the
second circuit, on the other hand,
is much more stable; it is inde-
pendent of optocoupler gain and
has no matched current sources
to worry about. However, the
second circuit requires two
optocouplers, separate gain
adjustments for the positive and
negative portions of the signal,
and can exhibit crossover distor-
tion near zero volts. The correct
circuit to choose for an applica-
tion would depend on the
requirements of that particular
application. As with the basic
isolation amplifier circuit in
Figure 12a, the circuits in Figure
14 are simplified and would
require a few additional compo-
nents to function properly. Two
example circuits that operate with
bipolar input signals are
discussed in the next section.
As a final example of circuit
design flexibility, the simplified
schematics in Figure 15 illustrate
how to implement 4-20 mA
analog current-loop transmitter
and receiver circuits using the
HCNR200/201 optocoupler. An
important feature of these circuits
is that the loop side of the circuit
is powered entirely by the loop
current, eliminating the need for
an isolated power supply.
The input and output circuits in
Figure 15a are the same as the
negative input and positive output
circuits shown in Figures 13c and
13b, except for the addition of R3
and zener diode D1 on the input
side of the circuit. D1 regulates
the supply voltage for the input
amplifier, while R3 forms a
current divider with R1 to scale
the loop current down from 20
mA to an appropriate level for the
input circuit (<50 µA).
As in the simpler circuits, the
input amplifier adjusts the LED
current so that both of its input
terminals are at the same voltage.
The loop current is then divided
1-431