Data Sheet

ADM3252E

Rev. 0 | Page 13 of 16

DC CORRECTNESS AND MAGNETIC FIELD

IMMUNITY

Positive and negative logic transitions at the isolator input cause

narrow (~1 ns) pulses to be sent to the decoder via the trans-

former. The decoder is bistable and is, therefore, either set or

reset by the pulses, indicating input logic transitions.

In the absence of logic transitions at the input for more than

1 µs, periodic sets of refresh pulses (indicative of the correct

input state) are sent to ensure dc correctness at the output. If the

decoder receives no internal pulses for more than approximately

5 µs, the input side 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

ADM3252E during power-up and power-down operations only.

The limitation on the ADM3252E magnetic field immunity is

set by the condition in which induced voltage in the receiving

coil of the transformer is sufficiently large to falsely set or reset

the decoder. The following analysis defines the conditions

under which this can occur.

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 estab-

lishing 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)

where:

β is the magnetic flux density (gauss).

N is the number of turns in the receiving coil.

Given the geometry of the receiving coil internally 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 21.

Figure 21. 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 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 >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 trans-

formers. Figure 22 expresses these allowable current magnitudes

as a function of frequency for selected distances. As shown in

Figure 22, the ADM3252E is extremely immune and can be

affected only by extremely large currents operated at high

frequency very close to the component. For example, at a

magnetic field frequency of 1 MHz, a 0.5 kA current placed

5 mm away from the ADM3252E is required to affect the

operation of the component.

Figure 22. Maximum Allowable Current

for Various Current-to-ADM3252E Spacings

Note that in the presence of strong magnetic fields and high

frequencies, any loops formed by PCB traces may induce error

voltages sufficiently large to trigger the thresholds of succeeding

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

possibility.

POWER CONSIDERATIONS

The ADM3252E power input, data input channels on the primary

side, and data channels on the secondary side are all protected from

premature operation by undervoltage lockout (UVLO) circuitry.

Below the minimum operating voltage, the power converter holds

its oscillator inactive and all input channel drivers and refresh cir-

cuits are idle. Outputs remain in a high impedance state to prevent

transmission of undefined states during power-up and power-

down operations.

MAGNETIC FIELD FREQUENCY (Hz)

100

0.001

1M

10

0.01

1k

10k

10M

0.1

1

100M

100k

MAGNETIC FIELD FREQUENCY (Hz)

1K

100

10

1

0.1

0.01

1k

10k

100M

100k

1M

10M

DISTANCE = 5mm

DISTANCE = 1m

DISTANCE = 100mm