Datasheet

Data Sheet ADM2582E/ADM2587E
Rev. F | 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)Σπr
n
2
; n = 1, 2, … , N
where:
β is magnetic flux density (gauss).
N is the number of turns in the receiving coil.
r
n
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
Figure 33.
MAGNETIC FIELD FREQUENCY (Hz)
100
MAXIMUM ALLOWABLE MAGNETIC FLUX
DENSITY (kGauss)
0.001
1M
10
0.01
1k 10k 10M
0.1
1
100M100k
08111-019
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)
MAXIMUM ALLOWABLE CURRENT (kA)
1k
100
10
1
0.1
0.01
1k 10k 100M100k 1M 10M
DISTANCE = 5mm
DISTANCE = 1m
DISTANCE = 100mm
08111-020
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.
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