Data Sheet

ADN4620/ADN4621

APPLICATIONS INFORMATION

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Rev. 0 | 21 of 24

Figure 36. Isolated Serial Communication Example (Isolated Serial Gigabit Ethernet Switch Cascaded Using ADN4621)

MAGNETIC FIELD IMMUNITY

The limitation on the magnetic field immunity of the device is set

by the condition in which the induced voltage in the transformer

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

the decoder. The following analysis defines such conditions. The

ADN4620/ADN4621 is examined in a 1.7 V operating condition

because this operating condition represents the most susceptible

mode of operation for these products.

The pulses at the transformer output have an amplitude greater

than 0.35 V. The decoder has a sensing threshold of about 0.11 V,

therefore establishing a 0.24 V margin in which induced voltages

are tolerated.

The voltage (V) induced across the receiving coil is given by

(1)

where:

dβ is the change in magnetic flux density.

dt is the change in time.

N is the number of turns in the receiving coil.

Given the geometry of the receiving coil in the ADN4620/ADN4621

and an imposed requirement that the induced voltage be, at most,

50% of the 0.11 V threshold at the decoder, a maximum allowable

external magnetic flux density is calculated as shown in Figure 37.

Figure 37. Maximum Allowable External Magnetic Flux Density

For example, at a magnetic field frequency of 1 MHz, the maximum

allowable magnetic field of 1.06 kgauss induces a voltage of 0.055

V at the receiving coil. This voltage is about 50% of the sensing

threshold and does not cause a faulty output transition. If such an

event occurs with the worst case polarity during a transmitted pulse,