ADuM1250/ADuM1251

Rev. 0 | Page 11 of 12

TYPICAL APPLICATION DIAGRAM

ADuM1250

1

2

3

4

8

7

6

5

MAGNETIC FIELD IMMUNITY

The ADuM1250 is extremely immune to external magnetic

fields. The limitation on the ADuM1250’s 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 ADuM1250 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

∑

=

Π

−

=

N

n

r

dt

d

V

n

...

,

2

,

1

;

)

/

β

(

2

where:

β is the magnetic flux density (gauss).

N is the number of turns in the receiving coil.

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.

MAGNETIC FIELD FREQUENCY (Hz)

100

0.001

1M

10

0.01

1k

10k

10M

0.1

1

100M

100k

Figure 10. 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 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

ADuM1250 transformers. Figure 11 expresses these allowable

current magnitudes as a function of frequency for selected

distances. As shown in Figure 11, the ADuM1250 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 ADuM1250 to affect the

component’s operation.

MAGNETIC FIELD FREQUENCY (Hz)

1000

100

10

1

0.1

0.01

1k

10k

100M

100k

1M

10M

DISTANCE = 5mm

DISTANCE = 1m

DISTANCE = 100mm

Figure 11. Maximum Allowable Current for Various

Current-to-ADuM1250 Spacings

Note that at combinations of strong magnetic fields and high

frequencies, any loops formed by printed circuit board traces

could 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.