Datasheet

ADM3251E Data Sheet
Rev. F | Page 14 of 16
The limitation on the ADM3251E magnetic field immunity is
set by the condition in which induced voltage in the receiving
coil of the transformer is sufficiently large to falsely set or reset
the decoder. The following analysis defines the conditions
under which this can occur.
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 estab-
lishing 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)
∑πr
n
2
; 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).
Given the geometry of the receiving coil internally 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 22.
MAGNETIC FIELD FREQUENCY (Hz)
100
MAXIMUM ALLOWABLE MAGNETIC FLUX
DENSITY (kgauss)
0.001
1M
10
0.01
1k 10k 10M
0.1
1
100M100k
07388-023
Figure 22. 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 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 of the worst-case polarity), the received pulse is
reduced 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 trans-
formers. Figure 23 expresses these allowable current magnitudes
as a function of frequency for selected distances. As shown in
Figure 23, the ADM3251E is extremely immune and can be
affected only by extremely large currents operated at high
frequency very close to the component. For example, at a
magnetic field frequency of 1 MHz, a 0.5 kA current placed
5 mm away from the ADM3251E is required to affect the
operation of the component.
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
07388-024
Figure 23. Maximum Allowable Current
for Various Current-to-ADM3251E Spacings
In the presence of strong magnetic fields and high frequencies, any
loops formed by PCB traces may induce error voltages sufficiently
large to trigger the thresholds of succeeding circuitry. Exercise care
in the layout of such traces to avoid this possibility.
ISOLATED POWER SUPPLY CIRCUIT
To operate the ADM3251E with its internal dc-to-dc converter
disabled, connect a voltage of between 3.0 V and 3.7 V to the
V
CC
pin and apply an isolated power of between 3.0 V and 5.5 V
to the V
ISO
pin, referenced to GND
ISO
.
A transformer driver circuit with a center-tapped transformer
and LDO can be used to generate the isolated supply, as shown
in Figure
24. The center-tapped transformer provides electrical
isolation of the 5 V power supply. The primary winding of the
transformer is excited with a pair of square waveforms that are
180° out of phase with each other. A pair of Schottky diodes and
a smoothing capacitor are used to create a rectified signal from
the secondary winding. The ADP3330 linear voltage regulator
provides a regulated power supply to the bus side circuitry
(V
ISO
) of the ADM3251E.
ADP3330
IN
NR
+
+
SD103C
22µF 10µF
5V
OUT
SD103C
78253
V
CC
V
CC
V
CC
GND
ISOLATION
BARRIER
SD
ERR
TRANSFORMER
DRIVER
V
CC
GND
V
ISO
GND
ISO
ADM3251E
07388-022
Figure 24. Isolated Power Supply Circuit