ADM2682E/ADM2687E

Preliminary Technical Data

Rev. PrE | Page 16 of 20

DC CORRECTNESS AND MAGNETIC FIELD IMMUNITY

The digital signals transmit across the isolation barrier using

i

Coupler 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 should occur in the ADM2682E/ADM2687E devices

only during power-up and power-down operations. The limitation

on the ADM2682E/ADM2687E 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 ADM2682E/ADM2687E

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 ADM2682E/

ADM2687E 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 33.

Figure 33. 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

ADM2682E/ADM2687E transformers. Figure 34 expresses

these allowable current magnitudes as a function of frequency

for selected distances. As shown in Figure 34, the ADM2682E/

ADM2687E are 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 ADM2682E/ADM2687E to affect

component operation.

Figure 34. Maximum Allowable Current for Various Current-to-

ADM2682E/ADM2687E Spacings

Note that in combinations of strong magnetic field and high

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

traces can induce error voltages sufficiently large to trigger the

thresholds of succeeding circuitry. Take care in the layout of

such traces to avoid this possibility.

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