Datasheet

ADM2485
Rev. A | Page 15 of 20
MAGNETIC FIELD IMMUNITY
Because iCouplers use a coreless technology, no magnetic
components are present and the problem of magnetic saturation
of the core material does not exist. Therefore, iCouplers have
essentially infinite dc field immunity. The following analysis
defines the conditions under which this can occur. The ADM2485
3.3 V operating condition is examined because it represents the
most susceptible mode of operation.
The limitation on the iCoupler ac magnetic field immunity is
set by the condition in which the induced error voltage in the
receiving coil (the bottom coil, in this case) is made sufficiently
large, either to falsely set or reset the decoder. The voltage
induced across the bottom coil is given by
∑
π
⎟
⎠
⎞
⎜
⎝
⎛
−
=
2
n
r
dt
dβ
V
; n = 1, 2 … N (1)
where, if the pulses at the transformer output are greater than
1.0 V in amplitude:
β is the magnetic flux density (gauss).
N is the number of turns in the receiving coil.
r
n
is the radius of nth turn in the receiving coil (cm).
The decoder has a sensing threshold of about 0.5 V; therefore,
there is a 0.5 V margin where induced voltages can be tolerated.
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 31.
MAGNETIC FIELD FREQUENCY (Hz)
1k 10k 100k 100M1M 10M
100
10
1
0.1
0.01
0.001
MAXIMUM ALLOWABLE MAGNETIC
FLUX DENSITY (kGAUSS)
0
6021-027
Figure 31. Maximum Allowable External Magnetic Flux Density vs.
Magnetic Field Frequency
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, which 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
it 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 32 shows the magnetic flux density values in terms of
more familiar quantities, such as maximum allowable current
flow at given distances from the ADM2485 transformers.
MAGNETIC FIELD FREQUENCY (Hz)
1k 10k 100k 100M1M 10M
DISTANCE = 1m
DISTANCE = 100mm
DISTANCE = 5mm
1000
100
0.1
0
10
0.01
MAXIMUM ALLOWABLE CURRENT (kA)
06021-028
Figure 32. Maximum Allowable Current for
Various Current-to-ADM2485 Spacings
At combinations of strong magnetic field and high frequency,
any loops formed by printed circuit board (PCB) traces could
induce sufficiently large error voltages to trigger the thresholds
of succeeding circuitry. Care must be taken in the layout of such
traces to avoid this possibility.