Avago Technologies ACPL-224-500E - Manual (Page 10)

AV02-4387EN 8 Avago Technologies
Figure 11. AC Equivalent Circuit for HCPL-261X.
Common-Mode Transient Rejection
Common-mode noise can enter
a system through conductive, in-
ductive, or capacitive coupling. An
example of a “conducted” noise
voltage is the difference in ground
potential that may exist between
two connected systems in a plant.
The two systems may experience a
small voltage difference between
their ground references. This voltage
difference might cause a ground-
loop current to flow. If the imped-
ance of the path through which the
ground-loop current flows is large
enough, a significant amount of in-
terference will result. Capaci tive or
inductive coupling may occur when
signal wires run close to ac power
cables. Electromag netically induced
interference (EMI) can also be
coupled from adjacent signal lines
or nearby equipment, especially in
factory environments. Other sources
of common-mode noise that can be
coupled into a system include light-
ning strikes and electrostatic dis-
charge (ESD).
Optical isolation is a useful tech nique
for reducing common-mode inter-
ference. Optocouplers, like trans-
formers and capacitively-coupled
devices, provide isolation between
the input and output of a system.
Transformers, by virtue of their high
primary-to-secondary capacitance,
tend to have lower CMTR capabil-
ity. Capacitively-coupled devices
tend to have poor CMTR capability
(since in these devices fast, transient
common-mode pulses pass across
the coupling capaci tor and are not
filtered out.) Optocouplers, having
low input-to-output capacitance,
typically provide better common-
mode rejection than transformers
or capacitively-coupled devices. The
CMR specification of an optocoup ler
ranges up to V
CM
= 1500 V amplitude
and up to 15,000 V/µs rate of change
of V
CM
, for high-CMR products.
Another advantage of optocoup-
lers lies in the area of EMI genera-
tion and susceptibility. Transformers
typically radiate electromagnetic in-
terference (EMI) and are susceptible
to magnetic fields. Capacitively-cou-
pled devices generate ground-loop
current, thus generating EMI. Opto-
couplers use light for data transmis-
sion; additionally, they effectively
eliminate ground-loop current.
Therefore, they do not radiate nor
are they affected by stray magnetic
fields. This ability is well-recog-
nized in the European Community
where systems designers need to
achieve system-level standards
(now adopted as EN50081/EN50082
which set limits on the amount of
acceptable EMI a system radiates or
to which it is immune.)
A technique which may be used to
further enhance CMTR is an “LED
split-resistor” technique as shown in
Figure 11; (note that the V
DM
which
would appear between the top and
bottom R
LED
s has not been shown in
this “ac equivalent circuit”). By using
two LED-resistors (instead of one)
the current change at the anode of
the LED is nearly canceled by the
current change at the cathode, thus
tending to keep the LED current
constant. This makes the optical iso-
lator more immune to CM transients
where C
LA
and C
LC
limit CMTR.
8
5
1
4
V
CC+
15 pF C
IC
SHIELD
+ –
V CM
6 3
7 2
1/2 R
LED
1/2 R LED
i
LN
i
LP
C
IA
V
O
GND
350 Ω
0.01 µF