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Data Sheet

ADE9103/ADE9112/ADE9113

APPLICATIONS INFORMATION

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Rev. A | 31 of 55

When using this feature, there could be a discontinuity in the

waveform when going through the previous steps to enable and to

disable the input shorting. The appropriate amount of settling time

based on the speed and accuracy required for the measurement

must be allowed.

MAGNETIC FIELD IMMUNITY OF ISOLATION

The ADE9112 and ADE9113 are immune to DC magnetic fields

because these devices use air core transformers. The limitation

on the ADE9112 and ADE9113 AC magnetic field immunity is set

by the condition in which the 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 is examined because

it is the nominal supply of the ADE9112 and ADE9113.

The pulses at the transformer output have an amplitude greater

than 1.0 V. The decoder has a sensing threshold at approximately

0.5 V, thus establishing a 0.5 V margin in which induced voltages

are tolerated. The voltage induced across the receiving coil is given

by the following:

V= −dBdt n=1Nπrn2

where:

B is the AC magnetic field: Β(t) = B × sin(ωt).

N is the number of turns in the receiving coil.

rn is the radius of the nth turn in the receiving coil.

Given the geometry of the receiving coil in the ADE9112/ADE9113

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

most 50% of the 0.5 V margin at the decoder, a maximum allowable

external magnetic field, B, is calculated (see the following equation

and Figure 41).

B= VTHR

2πf×n=1Nπrn2

where:

f is the frequency of the magnetic field.

B is the amplitude of the AC magnetic field.

Figure 41. Maximum Allowable External Magnetic Field

For example, at a magnetic field frequency of 10 kHz, the maximum

allowable magnetic field of 2.8 T induces a voltage of 0.25 V at the

receiving coil. This voltage 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 more than 1.0 V to 0.75

V, still well more than the 0.5 V sensing threshold of the decoder.

The preceding magnetic field values correspond to specific current

magnitudes at given distances from the ADE9112 and ADE9113

transformers.

I=Bμ0×2πd= V×d

μ0×f×n=1Nπrn2

where µ0 is 4π × 10−7 H/m, the magnetic permeability of the air.

Figure 42 expresses these allowable current magnitudes as a

function of frequency for selected distances. As shown in Figure 50,

the ADE9112/ADE9113 are extremely immune and can be affected

only by extremely large currents operating at high frequency close

to the component. For the 10 kHz example previously noted, a

current with an amplitude of 69 kA placed 5 mm from the ADE9112/

ADE9113 is required to affect component operation.

Note that at combinations of strong magnetic field and high fre-

quency, any loops formed by PCB traces can induce error voltages

large enough to trigger the thresholds of succeeding circuitry. Take

care in the layout of such traces to avoid this possibility (see the

Layout Guidelines section).

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