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ADM2481BRWZ

Half-Duplex, iCoupler Isolated RS-485 Transceiver

Analog Devices 

Given the geometry of the receiving coil and an imposed

requirement that the induced voltage is, at most, 50% of the

0.5 V margin at the decoder, a maximum allowable magnetic

field is calculated, as shown in Figure 27.

100.000

10.000

1.000

0.100

0.010

0.001

1k

10k

100k

1M

10M

MAGNETIC FIELD FREQUENCY (Hz)

100M

Figure 27. 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 the worst-case polarity, it reduces the received pulse from

>1.0 V to 0.75 V. This is well above the 0.5 V sensing threshold

of the decoder.

ADM2481

These magnetic flux density values are shown in Figure 28,

using more familiar quantities such as maximum allowable

current flow, at given distances away from the ADM2481

transformers.

1000.00

100.00

DISTANCE = 1m

DISTANCE = 5mm

10.00

DISTANCE = 100mm

1.00

0.10

0.01

1k

10k

100k

1M

10M

MAGNETIC FIELD FREQUENCY (Hz)

Figure 28. Maximum Allowable Current for Various

Current-to-ADM2481 Spacings

100M

At combinations of strong magnetic field and high frequency,

any loops formed by printed circuit board traces could induce

large enough error voltages to trigger the thresholds of succeed-

ing circuitry. To avoid this possibility, take care in the layout of

such traces.

Rev. 0 | Page 15 of 20

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