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Data subject to change. Copyright © 2005-2014 Avago Technologies. All rights reserved. Obsoletes AV01-0567EN
AV02-0886EN - July 1, 2014
The nal circuit shown in Figure 19 isolates a bipolar
analog signal using only one optocoupler and generates
two output signals: an analog signal proportional to the
magnitude of the input signal and a digital signal cor‑
responding to the sign of the input signal. This circuit is
especially useful for applica tions where the output of
the circuit is going to be applied to an analog‑to‑digital
converter. The primary advantages of this circuit are very
good linearity and oset, with only a single gain adjust‑
ment and no oset or balance adjustments.
To achieve very high linearity for bipolar signals, the
gain should be exactly the same for both positive and
negative input polarities. This circuit achieves excellent
linearity by using a single optocoupler and a single input
resistor, which guarantees identical gain for both posi‑
tive and negative polarities of the input signal. This pre‑
cise matching of gain for both polari ties is much more
dicult to obtain when separate components are used
for the dierent input polari ties, such as is the pre vious
circuit.
The circuit in Figure 19 is actually very similar to the pre‑
vious circuit. As mentioned above, only one optocoupler
is used. Because a photodiode can conduct current in
only one direction, two diodes (D1 and D2) are used to
steer the input current to the appropriate terminal of
input photodiode PD1 to allow bipolar input currents.
Normally the forward voltage drops of the diodes would
cause a serious linearity or accuracy problem. However,
an additional amplier is used to provide an appropriate
oset voltage to the other ampliers that exactly cancels
the diode voltage drops to maintain circuit accuracy.
Diodes D3 and D4 perform two dierent functions; the
diodes keep their respective ampliers active indepen‑
dent of the input signal polarity (as in the previous cir‑
cuit), and they also provide the feedback signal to PD1
that cancels the voltage drops of diodes D1 and D2.
Either a comparator or an extra op‑amp can be used to
sense the polarity of the input signal and drive an inex‑
pensive digital optocoupler, like a 6N139.
It is also possible to convert this circuit into a fully bipolar
circuit (with a bipolar output signal) by using the output
of the 6N139 to drive some CMOS switches to switch the
polarity of PD2 depending on the polarity of the input
signal, obtaining a bipolar output voltage swing.
HCNR200/201 SPICE Model
Figure 20 is the net list of a SPICE macro‑model for the
HCNR200/201 high‑linearity optocoupler. The macro‑
model accurately reects the primary characteristics of
the HCNR200/201 and should facilitate the design and
understanding of circuits using the HCNR200/201 opto‑
coupler.
