
ADM2582E/ADM2587E
Rev. C | Page 15 of 20
unpowered or nonfunctional, in which case, the isolator output
is forced to a default state by the watchdog timer circuit.
This situation should occur in the ADM2582E/ADM2587E devices
only during power-up and power-down operations. The limitation
on the ADM2582E/ADM2587E magnetic field immunity is set
by the condition in which induced voltage in the transformer
receiving coil is sufficiently large to either falsely set or reset the
decoder. The following analysis defines the conditions under
which this can occur.
The 3.3 V operating condition of the ADM2582E/ADM2587E
is examined because it represents the most susceptible mode of
operation. The pulses at the transformer output have an amplitude
of >1.0 V. The decoder has a sensing threshold of about 0.5 V,
thus establishing a 0.5 V margin in which induced voltages can
be tolerated. The voltage induced across the receiving coil is
given by
V = (dβ/dt)Σπrn2; n = 1, 2, … , N
where:
β is magnetic flux density (gauss).
N is the number of turns in the receiving coil.
rn is the radius of the nth turn in the receiving coil (cm).
Given the geometry of the receiving coil in the ADM2582E/
ADM2587E and an imposed requirement that the induced
voltage be, at most, 50% of the 0.5 V margin at the decoder, a
maximum allowable magnetic field is calculated as shown in
MAGNETIC FIELD FREQUENCY (Hz)
100
MA
XI
MU
M
A
L
O
W
A
B
L
E
MA
G
N
E
T
IC
F
L
U
X
DE
NS
IT
Y
(
kG
au
ss
)
0.001
1M
10
0.01
1k
10k
10M
0.1
1
100M
100k
08
11
1-
0
19
Figure 33. Maximum Allowable External Magnetic Flux Density
For example, at a magnetic field frequency of 1 MHz, the
maximum allowable magnetic field of 0.2 kgauss induces a
voltage of 0.25 V at the receiving coil. This is about 50% of the
sensing threshold and does not cause a faulty output transition.
Similarly, if such an event occurs during a transmitted pulse
(and is of the worst-case polarity), it reduces the received pulse
from >1.0 V to 0.75 V, which is still well above the 0.5 V sensing
threshold of the decoder.
The preceding magnetic flux density values correspond
to specific current magnitudes at given distances from the
ADM2582E/ADM2587E transformers.
Figure 34 expresses
these allowable current magnitudes as a function of frequency
for selected distances. As shown in
Figure 34, the ADM2582E/
ADM2587E are extremely immune and can be affected only by
extremely large currents operated at high frequency very close
to the component. For the 1 MHz example, a 0.5 kA current must
be placed 5 mm away from the ADM2582E/ADM2587E to affect
component operation.
MAGNETIC FIELD FREQUENCY (Hz)
M
AX
IM
UM
AL
L
O
W
ABL
E
CURR
E
NT
(k
A)
1k
100
10
1
0.1
0.01
1k
10k
100M
100k
1M
10M
DISTANCE = 5mm
DISTANCE = 1m
DISTANCE = 100mm
0
81
11
-02
0
Figure 34. Maximum Allowable Current for Various Current-to-
ADM2582E/ADM2587E Spacings
Note that in combinations of strong magnetic field and high
frequency, any loops formed by printed circuit board (PCB)
traces can induce error voltages sufficiently large to trigger the
thresholds of succeeding circuitry. Take care in the layout of
such traces to avoid this possibility.