Optimising high voltage common mode rejection performance of Avago ACPL-x6xL ultra-low power optocouplers
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Optimising high voltage common mode rejection
performance of Avago ACPL-x6xL ultra-low power
optocouplers
The ACPL-x6xL products are optically coupled
ultra-low power 10 MBd digital CMOS
optocouplers that operate with low power
consumption, especially when the light emitting
diode (LED) operates at low current. When a
current is applied to drive the LED, common
mode rejection (CMR) can be affected by
leakage paths inherent to the LED. To help deal
with the problem Avago has released a white
paper that describes the optimisation of CMR
performance with split limiting resistors at the
LED.
Introduction
Common mode noise is often seen at the system application level where there is a difference in
the ground levels of an isolating component’s input control circuitry and output control circuitry.
This is especially true when
a ground line is floating (device ground connected to a common line).
In the ACPL-x6xL family, the common mode rejection (CMR) specification indicates the ability to
reject common mode noise. This is also known as common mode transient rejection (CMTR).
CMTR describes the maximum tolerable rising/falling rate of a common mode voltage (given in
volts per microsecond, V/•s). The CMTR specification includes the amplitude of the common
mode voltage (VCM) that can be tolerated. The common mode voltage slew rate that the
optocoupler can tolerate and hold the correct output state is referred to as common mode
transient immunity (CMTI).
Common mode noise can be coupled to the optocoupler output by external circuitry. Common
mode noise, especially in a high electromagnetic interference (EMI) environment, can adversely
affect the output state of the
optocoupler through a conductive medium, primarily capacitive and inductive parasitics, Metallic
printed circuit board (PCB) tracks that operate at high frequency can couple charge to the LED
input pin or to the optocoupler output pin through parasitic capacitors between adjacent metal
tracks.
http://www.anglia.com/newsarchive/article.asp?article_id=4091&acrobatPrint=yes (1 of 2) [15/08/2012 09:25:36]
Optimising high voltage common mode rejection performance of Avago ACPL-x6xL ultra-low power optocouplers
It is often difficult to identify the root cause of common mode noise or interference that is
introduced by the circuit/system/application or by other forms of external factors that couple
noise. When the source of common mode noise is identified, corrective measures are easy to
implement by adding decoupling capacitors or filters to the system, or by adding some form of
shielding.
To read the entire White Paper, click here to download
For more information, or details on the full range of Avago products available from Anglia,
please email info@anglia.com
Alternatively, click on this link to go to the main Avago section where you can view other news
articles and product data.
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This news article was originally published in August 2012.
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8/11/11
http://www.anglia.com/newsarchive/article.asp?article_id=4091&acrobatPrint=yes (2 of 2) [15/08/2012 09:25:36]