ADM3053

Data Sheet

Rev. E | Page 14 of 18

DC CORRECTNESS AND MAGNETIC FIELD IMMUNITY

The digital signals transmit across the isolation barrier using

iCoupler technology. This technique uses chip-scale transformer

windings to couple the digital signals magnetically from one

side of the barrier to the other.

Digital inputs are encoded into waveforms that are capable of

exciting the primary transformer winding. At the secondary

winding, the induced waveforms are decoded into the binary

value that was originally transmitted.

Positive and negative logic transitions at the isolator input cause

narrow (~1 ns) pulses to be sent to the decoder via the transformer.

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

of 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 must occur in the ADM3053 devices only during

power-up and power-down operations. The limitation on the

ADM3053 magnetic field immunity is set by the condition in

which 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 of the ADM3053 is examined

because it represents the most susceptible mode of operation. 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 establishing a

0.5 V margin in which induced voltages can be tolerated. The

voltage induced across the receiving coil is given by

where:

β is magnetic flux density (gauss).

N is the number of turns in the receiving coil.

Given the geometry of the receiving coil in the ADM3053 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 26.

MAGNETIC FIELD FREQUENCY (Hz)

100

0.001

1M

10

0.01

1k

10k

10M

0.1

1

100M

100k

Figure 26. 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 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 >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

ADM3053 transformers. Figure 27 expresses these allowable

current magnitudes as a function of frequency for selected

distances. As shown in Figure 27, the ADM3053 is extremely

immune and can be affected only by extremely large currents

operated at high frequency very close to the component. For the

1 MHz example, a 0.5 kA current must be placed 5 mm away from

the ADM3053 to affect component operation.

MAGNETIC FIELD FREQUENCY (Hz)

1k

100

10

1

0.1

0.01

1k

10k

100M

100k

1M

10M

DISTANCE = 5mm

DISTANCE = 1m

DISTANCE = 100mm

Figure 27. Maximum Allowable Current for Various Current-to-ADM3053

Spacings

Note that in combinations of strong magnetic field and high

frequency, any loops formed by the printed circuit board (PCB)

traces can induce error voltages sufficiently large to trigger the

thresholds of succeeding circuitry. Proceed with caution in the

layout of such traces to prevent this from occurring.