Data Sheet ADM3054
Rev. D | Page 17 of 20
MAGNETIC FIELD IMMUNITY
The limitation on the magnetic field immunity of the iCoupler
is set by the condition in which an induced voltage in the receiving
coil of the transformer is large enough to either falsely set or
reset the decoder. The following analysis defines the conditions
under which this may occur. The 3 V operating condition of the
ADM3054 is examined because it represents the most susceptible
mode of operation.
The pulses at the transformer output have an amplitude greater
than 1 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
∑
π
β−
=
2
n
r
dt
d
V
; n = 1, 2, …, N
where:
β is the magnetic flux density (gauss).
N is the number of turns in the receiving coil.
r
n
is the radius of the n
th
turn in the receiving coil (cm).
Figure 30. Maximum Allowable External Magnetic Flux Density
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 can be determined using Figure 30.
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 approximately
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, still well above the 0.5 V sensing threshold
of the decoder.
Figure 31 shows the magnetic flux density values in terms of
more familiar quantities, such as maximum allowable current
flow at given distances away from the ADM3054 transformers.
Figure 31. Maximum Allowable Current for
Various Current-to-ADM3054 Spacings
With combinations of strong magnetic field and high frequency,
any loops formed by printed circuit board (PCB) traces can
induce error voltages large enough to trigger the thresholds of
succeeding circuitry. Therefore, care is necessary in the layout of
such traces to avoid this possibility.
MAGNETIC FIELD FREQUENCY (Hz)
1k 10k 100k 100M1M 10M
100
10
1
0.1
0.01
0.001
MAXIMUM ALLOWABLE MAGNETIC
FLUX DENSITY (kGAUSS)
10274-030
MAGNETIC FIELD FREQUENCY (Hz)
1k 10k 100k 100M1M 10M
DISTANCE = 1m
DISTANCE = 100mm
DISTANCE = 5mm
1000
100
0.1
1
10
0.01
MAXIMUM ALLOWABLE CURRENT (kA)
10274-031
