adum1251 Analog Devices, Inc., adum1251 Datasheet - Page 11

• Manufacturer Part Number: adum1251
• Description: Hot Swappable Dual I2c Isolators
• Manufacturer: Analog Devices, Inc.

TYPICAL APPLICATION DIAGRAM
MAGNETIC FIELD IMMUNITY
The ADuM125x is extremely immune to external magnetic
fields. The limitation on the ADuM125x magnetic field
immunity is set by the condition in which induced voltage in
the transformer’s receiving coil is sufficiently 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 ADuM125x is examined 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 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
where:
β is the magnetic flux density (gauss).
N is the number of turns in the receiving coil.
r
Given the geometry of the receiving coil in the ADuM1250 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 10.
n
is the radius of the nth turn in the receiving coil (cm).
GND
SDA
SCL
V
DD1
V
0.001
0.01
Figure 10. Maximum Allowable External Magnetic Flux Density
1
1
1
100
0.1
10
=
1
1k
Figure 9. Typical Isolated I
(
−
dβ
/
dt
1
2
4
3
10k
)
MAGNETIC FIELD FREQUENCY (Hz)
ADuM1250
∑
Π
r
n
2
100k
;
n
=
2
C Interface Using ADuM1250
, 1
8
7
6
5
, 2
...
1M
N
V
SDA
SCL
GND
DD2
2
2
2
10M
I
2
C BUS
100M
Rev. A | Page 11 of 12
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
(with the worst-case polarity), it reduces the received pulse
from >1.0 V to 0.75 V. Note that this 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 away from the
ADuM125x transformers. Figure 11 expresses these allowable
current magnitudes as a function of frequency for selected
distances. As shown in Figure 11, the ADuM125x is extremely
immune and can be affected only by extremely large currents
operated at high frequency and very close to the component.
For the 1 MHz example, one would have to place a 0.5 kA
current 5 mm away from the ADuM125x to affect the
component’s operation.
Note that at combinations of strong magnetic fields and high
frequencies, any loops formed by printed circuit board traces
can induce sufficiently large error voltages to trigger the
threshold of succeeding circuitry. Care should be taken in the
layout of such traces to avoid this possibility.
1000
0.01
100
0.1
10
1
1k
Figure 11. Maximum Allowable Current for Various
DISTANCE = 100mm
DISTANCE = 5mm
Current-to-ADuM125x Spacings
10k
MAGNETIC FIELD FREQUENCY (Hz)
100k
ADuM1250/ADuM1251
1M
DISTANCE = 1m
10M
100M