Avago Technologies ACPL-224-500E - Manual (Page 11)

AV02-4387EN 9 Avago Technologies
Dynamic CMR
Figure 13. High voltage common mode noise spike hardly causes a ripple in the output of 10MBd digital optocoupler ACPL-M61L.
Figure 12. Test Circuit for dynamic common mode rejection.
What is Dynamic CMR?
CMRL is used to denote CMR meas-
ured when output is logic LOW, and
CMRH for output logic HIGH. CMRL
/CMRH specification has been the
de facto standard when measuring
and comparing CMR performances
of isolators but this may not be ad-
equate.
CMRL /CMRH is measured under
static DC input/output condition,
i.e. the same condition when the
end application is in an idle, standby
mode. However, it is often more
critical to understand the CMR per-
formance when the system is active
with the isolators’ input and output
signals changing momentarily. The
Dynamic CMR Performance
The ultra low power 10MBd digital
CMOS optocoupler, ACPL-M61L, is
tested at VDD = 5V, with a 5MHz os-
cillator to simulate input data rate of
10MBd. The output did not show any
significant sign of disturbance even
after an injection of a high voltage
pulse measuring 48kV/ μs @ VCM
=1kV. This exceeded the 35kV/ μs @
VCM =1kV typical value specified in
the component datasheet. Figure 13
shows the scope shot taken during
the dynamic CMR measurement for
ACPL-M61L.
Output
Common Mode
Transient
Common Mode
Transient
Output
Dynamic CMR specification is de-
termined by measuring the CMR
performance when the inputs and
outputs are switching, thereby simu-
lating the actual operation of an iso-
lator. Figure 12 below shows the test
circuit to measure dynamic CMR.
The test setup is somewhat similar to
that for measuring static CMR except
that an oscillator chip is connect-
ed to provide the switching input
signal. A signal generator equip-
ment should never be used in place
of the oscillator as the injection of a
high common mode voltage could
lead to a destructive voltage spike
on the equipment’s GND line.