ADM2481 Datasheet, PDF (15/20 Pages) Analog Devices – Half-Duplex, iCoupler Isolated RS-485 Transceiver

Given the geometry of the receiving coil and an imposed
requirement that the induced voltage is, at most, 50% of the
0.5 V margin at the decoder, a maximum allowable magnetic
field is calculated, as shown in Figure 27.
100.000
10.000
1.000
0.100
0.010
0.001
1k
10k
100k
1M
10M
MAGNETIC FIELD FREQUENCY (Hz)
100M
Figure 27. 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 the worst-case polarity, it reduces the received pulse from
>1.0 V to 0.75 V. This is well above the 0.5 V sensing threshold
of the decoder.
ADM2481
These magnetic flux density values are shown in Figure 28,
using more familiar quantities such as maximum allowable
current flow, at given distances away from the ADM2481
transformers.
1000.00
100.00
DISTANCE = 1m
DISTANCE = 5mm
10.00
DISTANCE = 100mm
1.00
0.10
0.01
1k
10k
100k
1M
10M
MAGNETIC FIELD FREQUENCY (Hz)
Figure 28. Maximum Allowable Current for Various
Current-to-ADM2481 Spacings
100M
At combinations of strong magnetic field and high frequency,
any loops formed by printed circuit board traces could induce
large enough error voltages to trigger the thresholds of succeed-
ing circuitry. To avoid this possibility, take care in the layout of
such traces.
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