ADuM5401/ADuM5402/ADuM5403/ADuM5404
The limitation on the ADuM5401/ADuM5402/ADuM5403/
ADuM5404 magnetic field immunity is set by the condition in
which induced voltage in the transformer receiving coil is suffi-
ciently large to either falsely set or reset the decoder. The following
analysis defines the conditions under which this can occur.
PROPAGATION DELAY-RELATED PARAMETERS
Propagation delay is a parameter that describes the time it takes
a logic signal to propagate through a component (see Figure 24).
The propagation delay to a logic low output may differ from the
propagation delay to a logic high.
The 3.3 V operating condition of the ADuM5401/ADuM5402/
ADuM5403/ADuM5404 is examined because it represents the
most susceptible mode of operation.
INPUT (V
)
50%
Ix
tPLH
tPHL
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 estab-
lishing a 0.5 V margin in which induced voltages can be tolerated.
The voltage induced across the receiving coil is given by
OUTPUT (V
)
50%
Ox
Figure 24. Propagation Delay Parameters
Pulse width distortion is the maximum difference between
these two propagation delay values and is an indication of how
accurately the input signal timing is preserved.
2
V = (−dβ/dt)∑πrn ; n = 1, 2, … , N
where:
Channel-to-channel matching refers to the maximum amount
the propagation delay differs between channels within a single
ADuM5401/ADuM5402/ADuM5403/ADuM5404 component.
β is 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).
Propagation delay skew refers to the maximum amount the
propagation delay differs between multiple ADuM5401/
ADuM5402/ADuM5403/ADuM5404 components operating
under the same conditions.
Given the geometry of the receiving coil in the ADuM5401/
ADuM5402/ADuM5403/ADuM5404, and an imposed require-
ment 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 25.
EMI CONSIDERATIONS
100
The dc-to-dc converter section of the ADuM5401/ADuM5402/
ADuM5403/ADuM5404 components must, of necessity, operate
at very high frequency to allow efficient power transfer through
the small transformers. This creates high frequency currents that
can propagate in circuit board ground and power planes, causing
edge and dipole radiation. Grounded enclosures are recommended
for applications that use these devices. If grounded enclosures are
not possible, good RF design practices should be followed in layout
of the PCB. See www.analog.com for the most current PCB layout
recommendations specifically for the ADuM5401/ADuM5402/
ADuM5403/ADuM5404.
10
1
0.1
0.01
DC CORRECTNESS AND MAGNETIC FIELD IMMUNITY
0.001
1k
10k
100k
1M
10M
100M
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 (see Table 10) by the watchdog
timer circuit. This situation should occur in the ADuM5401/
ADuM5402/ADuM5403/ADuM5404 devices only during
power-up and power-down operations.
MAGNETIC FIELD FREQUENCY (Hz)
Figure 25. 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.
Rev. 0 | Page 18 of 24
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