Data Sheet

ADuM2250/ADuM2251

Rev. A | Page 11 of 16

STARTUP

feature that prevents the signal channels from operating unless

certain criteria is met. This feature is to avoid the possibility of

tently during power-up/power-down.

Criteria that must be met for the signal channels to be enabled

are as follows:



Both supplies must be at least 2.5 V.



At least 40 μs must elapse after both supplies exceed the

internal start-up threshold of 2.0 V.

Until both of these criteria are met for both supplies, the

ADuM2250/ADuM2251 outputs are pulled high thereby

ensuring a startup that avoids any disturbances on the bus.

Figure 8 and Figure 9 illustrate the supply conditions for fast

and slow input supply slew rates.

SUPPLY VALID

MINIMUM RECOMMENDED

OPERATING SUPPLY, 3.0V

MINIMUM VALID SUPPLY, 2.5V

INTERNAL STARTUP

THRESHOLD, 2.0V

40µs

Figure 8. Start-Up Condition, Supply Slew Rate < 12.5 V/ms

SUPPLY VALID

MINIMUM RECOMMENDED

OPERATING SUPPLY, 3.0V

MINIMUM VALID SUPPLY, 2.5V

INTERNAL STARTUP

THRESHOLD, 2.0V

40µs

Figure 9. Start-Up Condition, Supply Slew Rate > 12.5 V/ms

MAGNETIC FIELD IMMUNITY

The ADuM2250/ADuM2251 are extremely immune to external

magnetic fields. The limitation on the magnetic field immunity

of the ADuM2250/ADuM2251 is set by the condition in which

induced voltage in the receiving coil of the transformer is suffi-

ciently 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 ADuM2250/ADuM2251 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,

therefore 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

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 ADuM2250/

ADuM2251 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 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 were to occur during a transmitted

pulse (and had the worst-case polarity), it would reduce the

received pulse from >1.0 V to 0.75 V (still well above the 0.5 V

sensing threshold of the decoder).