ADE7932 datasheet(78/120 Pages) AD

78 / 120 page

ADE7978/ADE7933/ADE7932

Data Sheet

Rev. 0 | Page 78 of 120

MAGNETIC FIELD IMMUNITY

Because the ADE7933/ADE7932 use air core transformers, the

devices are immune to dc magnetic fields. The limitation on the

ac magnetic field immunity of the ADE7933/ADE7932 is set by

the condition in which the induced voltage in the receiving coil

of the transformer 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 ADE7933/ADE7932.

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 within which induced volt-

ages can be tolerated. The voltage induced across the receiving

coil is given by

∑













−

(53)

where:

β 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 ADE7933/

ADE7932 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 magnetic field (B) is calculated, as shown

in Figure 97 and Equation 54.

∑

THR

(54)

where f is the frequency of the magnetic field.

100

0.1

0.01

0.001

10k

100k

10M

100M

FREQUENCY (Hz)

Figure 97. Maximum Allowable External Magnetic Field

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

mum allowable magnetic field of 2.8 T induces a voltage of 0.25 V

at the receiving coil. This voltage is approximately 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), the received pulse is reduced 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 ADE7933/ADE7932

transformers.

∑

(55)

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

Figure 98 shows the allowable current magnitudes as a function

of frequency for selected distances. As shown in Figure 98, the

ADE7933/ADE7932 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, a current with

a magnitude of 69 kA must be placed 5 mm away from the

ADE7933/ADE7932 to affect the operation of the component.

1000

100

0.1

0.01

10k

100k

10M

100M

FREQUENCY (Hz)

0.005m

0.1m

Figure 98. Maximum Allowable Current for Various Current-to-

ADE7933/ADE7932 Spacings

Note that at 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. Exercise care in the layout of such traces to avoid this

possibility (see the Layout Guidelines section).

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