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AV02-4059EN - April 19, 2013
Optocoupler CMR Performance
The principal protection against common mode noise,
comes from the fundamental isolation properties of the
optocoupler, and this in turn is directly related to the In-
put-Output leakage capacitance of the optocoupler.
To provide maximum protection to circuitry connected
to the input or output of the optocoupler the leakage
capac¬itance is minimized by having large separation
distances at all points in the optocoupler construction, in-
cluding the LED/photodiode interface.
In addition to the optocouplers basic physical
construc¬tion, additional circuit design steps mitigate
the eects of common mode noise. The most important
of these is the Faraday shield on the photodetector stage.
A Faraday shield is eective in optocouplers because
the internal modulation frequency (light) is many orders
of magnitude higher than the common mode noise fre-
quency.
Improving CMR Performance at the Application Level
In an end application it desirable that the optocouplers
common mode isolation be as close as possible to that
indicated in the data sheet specications. The rst step in
meeting this goal is to ensure maximum separation be-
tween PCB interconnects on either side of the opto-cou-
pler is maintained and that PCB tracks beneath the opto-
coupler are avoided.
It is inevitable that a certain amount of CMR noise will be
coupled into the inputs and this can potentially result in
false-triggering of the input. This problem is frequently
observed in devices with input high input impedance. In
some cases this can cause momentary missing pulses and
may even cause input circuitry to latch-up in some alter-
nate technologies.
The ACPL-M62L optocoupler family does not have an in-
put latch-up issue. Even at very high CMR levels such as
those experienced in end equipment level tests (for ex-
ample IEC61000-4-4) the ACPL-M62L series is immune to
latch-up because of the simple diode structure of the LED.
In some cases achieving the rated data sheet CMR
per¬formance level is not possible in an application. This
is often because of the practical need to actually connect
the isolator input to the output of a dynamically chang-
ing signal rather than tying the input statically to VDD or
GND. A data sheet CMR “specmanship” issue is often seen
with alternative technology isolators that are based on AC
encoding techniques.
To address the need to dene achievable end application
performance on data sheets, the ACPL-M62L optocouplers
include an additional typical performance specication
for dynamic CMR in the electrical parameter table. The dy-
namic CMR specication indicates the typical achievable
CMR performance as the input is being toggled on or o
during a CMR transient.
The logic output the ACPL-M62L optocouplers is mainly
controlled by LED current level, and since the LED current
features very fast rise and fall times, dynamic noise immu-
nity is essentially the same as static noise immunity.
Despite their immunity to input latch-up and the excel-
lent dynamic CMR immunity, ACPL-M62L opto¬coupler
devices are still potentially vulnerable to miss-operation
caused by the LED being turned either on or o during a
CMR disturbance. If the LED status could be ensured by
design, the overall application level CMR performance
would be that of the photodetector. To benet from the
inherently high CMR capabilities of the ACPL-M62L family,
some simple steps about operating the LED at the appli-
cation level should be taken.
In particular, ensure that the LED stays either on or o dur-
ing a CMR transient. Some common design techniques to
accomplish this are:
• Keep the LED on:
– Overdrive the LED with a higher than required
forward current.
• Keep the LED o:
During the O state:
i) Reverse bias the LED.
ii) Minimize the o-state impedance across the anode
and cathode of the LED.
All these methods allow the full CMR capability of the AC-
PL-M62L family to be achieved, but they do have practical
implementation issues or require a compromise on power
consumption.
There is, however, an eective method to meet the goal
of maintaining the LED status during a CMR event with
no other design compromises other than adding a single
resistor.
This CMR optimization takes advantage of the dierential
connection to the LED. By ensuring the common mode
impedances at both the cathode and anode of the LED
are equal, the CMR transient on the LED is eectively can-
celed. As shown in Figure 9, this is easily achieved by using
two, instead of one, input bias resistors.
