Avago Technologies ACPL-224-500E - Manual (Page 6)

AV02-4387EN 4 Avago Technologies
Figure 5. Illustration of V
CM
Common-Mode Pulse. Figure 6a. Interference Circuit Model.
Circuit designers often encounter
the adverse effects of common-
mode noise in a design. Once a
common-mode problem is identi-
fied, there are several ways that it
can be resolved. However, common-
mode interference manifests itself in
many ways; therefore, it may be hard
to determine whether it is the cause
of a circuit’s misbehavior. If a system
is connected and running but only
produces erroneous data, common-
mode noise may be the reason.
This section describes sources of
common-mode problems, presents
possible solutions, and highlights
the technology that Avago Tech-
nologies' Components Group uses
to produce opto-isolators with su-
perior Common-Mode Performance.
Common-mode rejection (CMR) is
a measure of the ability of a device
to tolerate common-mode noise.
Avago specifies common-mode
rejection as common-mode transient
rejection (CMTR). CMTR describes the
maximum tolerable rate-of-rise (or
fall) of a common-mode voltage
(given in volts per micro second). The
specification for CMTR also includes
the amplitude of the common-mode
voltage (V
CM
) that can be tolerated.
Common-mode interference that
exceeds the maximum specifica tion
might result in abnormal voltage
transitions or excessive noise on the
output signal. (CMTR is slightly dif-
ferent than common-mode rejec-
tion ratio CMRR, often used for analog
devices and commonly specified in
dB as the ratio of the differential-
mode gain to the common-mode
gain.)
Avago optocouplers rely on two
key technical strengths to achieve
high CMTR. The first is use of a pro-
prietary, low-cost Faraday shield
which decouples the optocoupler
input side from the output side.
The second method is by unique
package design which minimizes
input-to-output capacitance. The
importance of these two strengths is
explained as follows.
Figure 5 illustrates a Common-mode
transient pulse (V
CM
).
Figure 6a and 6b show interfer-
ence circuit models for two types
of possible common-mode failure
mechanisms for a single-transistor
optocoupler. The dashed lines are
shown to indicate external compo-
nents added to the optocoupler. V
CM
represents a voltage spike across the
optocoupler isolation path between
the output-side ground (V
G2
) and
input-side ground (V
G1
). V
DM
repre-
sents a signal voltage applied across
the input side.
R
L
+ (V CM ) –
OPTO-
ISOLATOR
COMMON-MODE TRANSIENT
R
V
TRANSMIT SIDE RECEIVE SIDE
VOLTMETER
LED
V
DM
V
G1
INPUT GROUND
V G2
OUTPUT GROUND
C SB C IS
i
CM
I
B
I
P
PHOTODIODE
V O
R L
INTERNAL SHIELD
NOTE: i CM GETS DIVERTED TO GROUND, V G2 , WHEN INPUT IS OFF.
i CM IS SUPPLIED FROM GROUND, V G2 , WHEN OUTPUT IS ON.
V
CM
VOLTMETER
Common-Mode Transient Rejection
Introduction