ADE7913 datasheet(32/44 Pages) AD

32 / 44 page

ADE7912/ADE7913

Data Sheet

Rev. 0 | Page 32 of 44

MAGNETIC FIELD IMMUNITY

The ADE7912/ADE7913 are immune to dc magnetic fields

because they use air core transformers. The limitation on the

ADE7912/ADE7913 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 ADE7912/ADE7913.

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

∑











 −

(12)

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 ADE7912/

ADE7913 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, as shown in Equation 13 and Figure 50.

∑

THR

(13)

where:

f is the frequency of the magnetic field.

B is the amplitude of the ac magnetic field.

Figure 50. 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 above the 0.5 V

sensing threshold of the decoder.

The preceding magnetic field values correspond to specific

current magnitudes at given distances from the ADE7912/

ADE7913 transformers.

∑

(14)

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

Figure 51 expresses these allowable current magnitudes as a

function of frequency for selected distances. As shown in Figure 51,

the ADE7912/ADE7913 are extremely immune and can be

affected only by extremely large currents operated at high

frequency very close to the component. For the 10 kHz example

previously noted, a current with an amplitude of 69 kA placed

mm from the ADE7912/ADE7913 is required to affect

component operation.

Note that at combinations of strong magnetic field and high

frequency, any loops formed by PCB traces can induce error

voltages sufficiently 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).

Figure 51. Maximum Allowable Current for Various Current-to-

ADE7912/ADE7913 Spacings

100

0.1

0.01

0.001

10k

100k

10M

100M

FREQUENCY (Hz)

1000

100

0.1

0.01

10k

100k

10M

100M

FREQUENCY (Hz)

0.005m

0.1m

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