ADuM1250/ADuM1251

Rev. D | Page 11 of 12

TYPICAL APPLICATION DIAGRAM

ADuM1250

1

2

3

4

8

7

6

5

OPTIONAL

200Ω

Figure 9 shows a typical application circuit including the pull-up

resistors required for both Side 1 and Side 2 busses. Bypass capacitors

is required for latch-up immunity if the ambient temperature

can be between 105°C and 125°C.

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











N

n

r

dt

dβ

V

n

...

,

2

,

1

;

)

/

(

2

where:

β

is the magnetic flux density (gauss).

N

is the number of turns in the receiving coil.

is the radius of the nth turn in the receiving coil (cm).

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

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.

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-ADuM125x Spacings

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.