ADUM1250(Rev0) Просмотр технического описания (PDF) - Analog Devices
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ADUM1250 Datasheet PDF : 12 Pages
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TYPICAL APPLICATION DIAGRAM
VDD
SDA1
SCL1
GND1
1 ADuM1250 8
2
7
3
6
4
5
V2
SDA2
SCL2
GND2
I2C BUS
Figure 9. Typical Isolated I2C Interface using ADuM1250
MAGNETIC FIELD IMMUNITY
The ADuM1250 is extremely immune to external magnetic
fields. The limitation on the ADuM1250’s magnetic field
immunity is set by the condition in which induced voltage in
the transformer’s receiving coil is sufficiently large to either
falsely set or reset the decoder. The following analysis defines
the conditions under which this may occur. The 3 V operating
condition of the ADuM1250 is examined because it represents
the most susceptible mode of operation.
The pulses at the transformer output have an amplitude greater
than 1.0 V. The decoder has a sensing threshold at 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)∑ Πrn2 ; n = 1, 2, ...N
where:
β is the magnetic flux density (gauss).
N is the number of turns in the receiving coil.
rn is the radius of the nth turn in the receiving coil (cm).
Given the geometry of the receiving coil in the ADuM1250 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 10.
100
10
1
0.1
0.01
0.001
1k
10k
100k
1M
10M
100M
MAGNETIC FIELD FREQUENCY (Hz)
Figure 10. Maximum Allowable External Magnetic Flux Density
ADuM1250/ADuM1251
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
(with the worst-case polarity), it reduces the received pulse
from > 1.0 V to 0.75 V. Note that this 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 away from the
ADuM1250 transformers. Figure 11 expresses these allowable
current magnitudes as a function of frequency for selected
distances. As shown in Figure 11, the ADuM1250 is extremely
immune and can be affected only by extremely large currents
operated at high frequency and very close to the component.
For the 1 MHz example, one would have to place a 0.5 kA
current 5 mm away from the ADuM1250 to affect the
component’s operation.
1000
DISTANCE = 1m
100
10
DISTANCE = 100mm
1
DISTANCE = 5mm
0.1
0.01
1k
10k
100k
1M
10M
100M
MAGNETIC FIELD FREQUENCY (Hz)
Figure 11. Maximum Allowable Current for Various
Current-to-ADuM1250 Spacings
Note that at combinations of strong magnetic fields and high
frequencies, any loops formed by printed circuit board traces
could induce sufficiently large error voltages to trigger the
threshold of succeeding circuitry. Care should be taken in the
layout of such traces to avoid this possibility.
Rev. 0 | Page 11 of 12
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