Avago Technologies ACPL-224-500E - Manual (Page 7)

AV02-4387EN 5 Avago Technologies
Common-Mode Transient Rejection
Figure 6b. Interference Circuit Model.
Referring to Figure 6a the para sitic
distributed capacitance C
IS
, which
might tend to couple transient
current into the transistor base node
(for example when the transistor is
in the “off” state) terminates on an
internal Faraday shield. Therefore
the transient current, I
CM
, gets di-
verted to output ground (V
G2
). Re-
ferring to Figure 6b, the parasitic
distrib uted capacitances, C
IA
and C
IC
are shown across the LED anode-to-
ground (V
G2
) and LED cathode-to-
ground (V
G2
) respectively. Because
the LED anode is at a relatively
higher impedance than the cathode
(i.e., R
LED
to ground) current at this
point will tend to be modulated
slightly during CM transients. For in-
stance, if the LED is on, then during
a positive transient (i.e., dV
CM
/dt > 0)
current will be diverted away from
the LED. For fast enough transients,
this may turn the LED off. (If R
LED
is
connected to the LED cathode side
then C
IC
provides a parasitic path
to divert current towards or away
from the LED.) This type of failure is
avoided by ensuring that C
IA
and C
IC
are small.
Figure 7 shows the possible effect
on the output voltage level of an op-
tocoupler due to a common-mode
pulse. The output is shown (success-
ively) in the high and low states.
(This might be observed if R
led
were
con nected as in Figures 6a, 6b.)
Figure 7. Common Mode Interference Effect.
LED
V
DM
V G1
INPUT GROUND
V G2
OUTPUT GROUND
C
IA
C IC
I B
I P
PHOTODIODE
V
O
R L
INTERNAL SHIELD
NOTE: CURRENT “STOLEN” FROM LED: i
LP = C IA
V CM
dV
CM
dt
i
LP
VOLTMETER
V CM
V OH
V OL
t
TRANSIENT
t
TRANSIENT
t
V IH
V
IL
V O
TRANSIENT RATE OF CHANGE =
V CM
t TRANSIENT
TRANSIENT AMPLITUDE = V CM
t