adm2587e Analog Devices, Inc., adm2587e Datasheet - Page 13

• Manufacturer Part Number: adm2587e
• Description: Isolated 15kv Esd Protected Rs-485 Transceiver With Integrated Isolated Dc/dc Converter
• Manufacturer: Analog Devices, Inc.

Preliminary Technical Data
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 ADM2582E/ADM2587E
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
V = (−dβ/dt)
where:
N is the number of turns in the receiving coil.
r
Given the geometry of the receiving coil in the ADM2582E/
ADM2587E, 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
Error! Reference source not found..
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
ADM2582E/ADM2587E transformers. Figure 26 expresses
these allowable current magnitudes as a function of frequency
for selected distances. As shown in Figure 26, the ADM2582E/
ADM2587E 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
β is magnetic flux density (gauss).
n
is the radius of the n
0.001
0.01
100
0.1
10
Figure 13. Maximum Allowable External Magnetic Flux Density
1
1k
Σ
πr
10k
n2
; n = 1, 2, … , N
MAGNETIC FIELD FREQUENCY (Hz)
th
turn in the receiving coil (cm).
100k
1M
10M
100M
Rev. PrH | Page 13 of 16
would need to be placed 5 mm away from the ADM2582E/
ADM2587E to affect component operation.
Note that in combinations of strong magnetic field and high
frequency, any loops formed by printed circuit board traces
could induce error voltages sufficiently large to trigger the
thresholds of succeeding circuitry. Care should be taken in the
layout of such traces to avoid this possibility.
0.01
100
0.1
1k
10
1
Figure 14. Maximum Allowable Current for Various Current-to-
1k
DISTANCE = 100mm
10k
DISTANCE = 5mm
ADM2582E/ADM2587E Spacings
MAGNETIC FIELD FREQUENCY (Hz)
100k
ADM2582E/ADM2587E
1M
DISTANCE = 1m
10M
100M