Datasheet

17
to worry about. However, the second circuit requires two
optocouplers, separate gain adjustments for the posi‑
tive and negative portions of the signal, and can exhibit
crossover distortion near zero volts. The correct circuit to
choose for an application would depend on the require‑
ments of that particular application. As with the basic
isolation amplier circuit in Figure 12a, the circuits in Fig‑
ure 14 are simplied and would require a few additional
components to function properly. Two example circuits
that operate with bipolar input signals are discussed in
the next section.
As a nal example of circuit design exibility, the simpli‑
ed schematics in Figure 15 illustrate how to implement
4‑20 mA analog current‑loop transmitter and receiver
circuits using the HCNR200/201 optocoupler. An impor‑
tant feature of these circuits is that the loop side of the
circuit is powered entirely by the loop current, eliminat‑
ing 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 regu‑
lates the supply voltage for the input amplier, 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 amplier adjusts the
LED current so that both of its input terminals are at the
same voltage. The loop current is then divided
between R1 and R3. I
PD1
is equal to the current in R1 and
is given by the following equation:
I
PD1
= I
LOOP
*R3/(R1+R3).
Combining the above equation with the equations used
for Figure 12a yields an overall expression relating the
output voltage to the loop current,
V
OUT
/I
LOOP
= K*(R2*R3)/(R1+R3).
Again, you can see that the relationship is constant, lin‑
ear, and independent of the characteristics of the LED.
The 4‑20 mA transmitter circuit in Figure 15b is a little dif‑
ferent from the previous circuits, particularly the output
circuit. The output circuit does not directly generate an
output voltage which is sensed by R2, it instead uses Q1
to generate an output current which ows through R3.
This output current generates a voltage across R3, which
is then sensed by R2. An analysis similar to the one above
yields the following expression relating output current
to input voltage:
I
LOOP
/V
IN
= K*(R2+R3)/(R1*R3).
Circuit Design Flexibility
Circuit design with the HCNR200/201 is very exible
because the LED and both photodiodes are accessible
to the designer. This allows the designer to make perf‑
ormance trade‑os that would otherwise be dicult to
make with commercially available isolation ampliers
(e.g., bandwidth vs. accuracy vs. cost). Analog isolation
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
simplied circuit topologies illustrating the design ex‑
ibility of the HCNR200/201 are discussed below.
The circuit in Figure 12a is congured to be non‑invert‑
ing 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 congurations as well. Figure 13 illustrates
how to change the basic circuit to accommodate both
positive and negative input and output voltages. The in‑
put and output circuits can be matched to achieve any
combination of positive and negative voltages, allowing
for both inverting and non‑inverting circuits.
All of the congurations described above are unipolar
(single polarity); the circuits cannot accommodate a sig‑
nal that might swing both positive and negative. It is pos‑
sible, however, to use the HCNR200/201 optocoupler to
implement a bipolar isolation amplier. Two topologies
that allow for bipolar operation are shown in Figure 14.
The circuit in Figure 14a uses two current sources to
oset the signal so that it appears to be unipolar to the
optocoupler. Current source I
OS1
provides enough oset
to ensure that I
PD1
is always positive. The second current
source, I
OS2
, provides an oset of opposite polarity to ob‑
tain a net circuit oset 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 rst optocoupler handles the pos‑
itive voltage excursions, while the second optocoupler
handles the negative ones. The output photodiodes are
connected in an antiparallel conguration so that they
produce output signals of opposite polarity.
The rst circuit has the obvious advantage of requiring
only one optocoupler; however, the oset 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 optocoupler will directly af‑
fect the oset of the circuit.
The oset performance of the second circuit, on the
other hand, is much more stable; it is independent of
optocoupler gain and has no matched current sources