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ADuM1250/ADuM1251
MAGNETIC FIELD IMMUNITY
The ADuM1250 / ADuM1251 are extremely immune to external
magnetic fields. The limitation on the magnetic field immunity
of the ADuM1250 / ADuM1251 is set by the condition in which
induced voltage in the receiving coil of the transformer is suffi-
ciently large 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 ADuM1250 / ADuM1251 is exam-
ined because it represents the most susceptible mode of operation.
The pulses at the transformer output have an amplitude greater
than 1.0 V. The decoder has a sensing threshold at approximately
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 ) ∑ π r n 2 ; n = 1, 2, … , N
where:
β is the magnetic flux density (gauss).
Data Sheet
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 voltage is approxi-
mately 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—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 away from the
ADuM1250 / ADuM1251 transformers. Figure 11 expresses these
allowable current magnitudes as a function of frequency for
selected distances. As shown in Figure 11, the ADuM1250 /
ADuM1251 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
placed 5 mm away from the ADuM1250 / ADuM1251 is required
to affect the operation of the component.
r n is the radius of the n th turn in the receiving coil (cm).
N is the total number of turns in the receiving coil.
Given the geometry of the receiving coil in the ADuM1250 /
ADuM1251 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 10.
100
10
1000
100
10
1
0.1
DISTANCE = 100mm
DISTANCE = 5mm
DISTANCE = 1m
1
0.01
1k
10k
100k
1M
10M
100M
MAGNETIC FIELD FREQUENCY (Hz)
0.1
0.01
Figure 11. Maximum Allowable Current for Various
Current-to- ADuM1250 / ADuM1251 Spacings
Note that at combinations of strong magnetic field and high
frequency, any loops formed by PCB traces can induce error
voltages sufficiently large to trigger the thresholds of succeeding
0.001
1k
10k 100k 1M 10M
MAGNETIC FIELD FREQUENCY (Hz)
100M
circuitry. Exercise care in the layout of such traces to avoid this
possibility.
Figure 10. Maximum Allowable External Magnetic Flux Density
Rev. G | Page 12 of 16
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