Likewise for common-mode transients which occur
when the LED is oﬀ (i.e. CMR
, since the output is “high”),
if an imbalance between I
equal to or greater than the switching threshold of the
optocoupler, the transient “signal” may cause the output
to spike below 2 V (which consti tutes a CMR
By using the recommended circuit in Figure 13, good
CMR can be achieved. The balanced I
help equalize I
to reduce the amount by which
is modulated from transient coupling through C
CMR with Other Drive Circuits
CMR performance with drive circuits other than that
shown in Figure 13 may be enhanced by following these
1. Use of drive circuits where current is shunted from the
LED in the LED “oﬀ” state (as shown in Figures 15 and
16). This is beneﬁcial for good CMR
2. Use of I
> 3.5 mA. This is good for high CMR
Figure 15 shows a circuit which can be used with any
totem-pole-output TTL/LSTTL/HCMOS logic gate. The
buﬀer PNP transistor allows the circuit to be used with
logic devices which have low current-sinking capability.
It also helps maintain the driving-gate power-supply cur-
rent at a constant level to minimize ground shifting for
other devices connected to the input-supply ground.
When using an open-collector TTL or open-drain CMOS
logic gate, the circuit in Figure 16 may be used. When
using a CMOS gate to drive the optocoupler, the circuit
shown in Figure 17 may be used. The diode in parallel
with the R
speeds the turn-oﬀ of the optocoupler
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Data subject to change. Copyright © 2005-2010 Avago Technologies. All rights reserved. Obsoletes AV01-0581EN
AV02-0616EN - February 8, 2010
results in a transient
Figure 15. TTL interface circuit.
Figure 16. TTL open-collector/open drain gate drive circuit.
Figure 17. CMOS gate drive circuit.