ADM2682EBRIZ datasheet(17/24 Pages) AD

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Data Sheet

ADM2682E/ADM2687E

Rev. C | Page 17 of 24

is assumed to be unpowered or nonfunctional, in which case,

the isolator output is forced to a default state by the watchdog

timer circuit.

This situation should occur in the ADM2682E/ADM2687E devices

only during power-up and power-down operations. The limitation

on the ADM2682E/ADM2687E magnetic field immunity is set

by the condition in which induced voltage in the transformer

receiving coil is sufficiently large to either falsely set or reset the

decoder. The following analysis defines the conditions under

which this can occur.

The 3.3 V operating condition of the ADM2682E/ADM2687E

is examined because it represents the most susceptible mode of

operation. 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 establishing 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)Σπrn2; n = 1, 2, … , N

where:

β is magnetic flux density (gauss).

N is the number of turns in the receiving coil.

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

Given the geometry of the receiving coil in the ADM2682E/

ADM2687E 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 39.

MAGNETIC FIELD FREQUENCY (Hz)

100

0.001

0.01

10k

10M

0.1

100M

100k

Figure 39. 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

(and is of the worst-case polarity), it reduces the received pulse

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 ADM2682E/

ADM2687E transformers. Figure 40 expresses these allowable

current magnitudes as a function of frequency for selected

distances. As shown in Figure 40, the ADM2682E/ADM2687E

are extremely immune and can be affected only by extremely

large currents operated at high frequency very close to the

component. For the 1 MHz example, a 0.5 kA current must be

placed 5 mm away from the ADM2682E/ADM2687E to affect

component operation.

MAGNETIC FIELD FREQUENCY (Hz)

100

0.1

0.01

10k

100M

100k

10M

DISTANCE = 5mm

DISTANCE = 1m

DISTANCE = 100mm

Figure 40. Maximum Allowable Current for Various Current-to-

ADM2682E/ADM2687E Spacings

Note that in combinations of strong magnetic field and high

frequency, any loops formed by PCB traces can induce error

voltages sufficiently large to trigger the thresholds of succeeding

circuitry. Take care in the layout of such traces to avoid this

possibility.

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