Datasheet

ADuM4160 Data Sheet
Rev. D | Page 12 of 16
The limitation on the magnetic field immunity of the ADuM4160
is set by the condition in which induced voltage in the receiving
coil of the transformer 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
ADuM4160 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 of about 0.5 V, thus
establishing a 0.5 V margin in which induced voltages are tolerated.
The voltage induced across the receiving coil is given by
V = (−dβ/dt)
∑∏r
n
2
; n = 1, 2, … , N
where:
β is magnetic flux density (gauss).
N is the number of turns in the receiving coil.
r
n
is the radius of the n
th
turn in the receiving coil (cm).
Given the geometry of the receiving coil in the ADuM4160 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 6.
MAGNETIC FIELD FREQUENCY (Hz)
MAXIMUM ALLOWABLE MAGNETIC FLUX
DENSITY (kguass)
1k
0.001
100
100M
10
1
0.1
0.01
10k 100k 1M 10M
08171-006
Figure 6. Maximum Allowable External Magnetic Flux Density
For example, at a magnetic field frequency of 1 MHz, the max-
imum 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—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 ADuM4160
transformers. Figure 7 expresses these allowable current
magnitudes as a function of frequency for selected distances.
MAGNETIC FIELD FREQUENCY (Hz)
MAXIMUM ALLOWABLE CURRENT (kA)
1000
100
10
1
0.1
0.01
1k 10k 100M100k 1M 10M
DISTANCE = 5mm
DISTANCE = 1m
DISTANCE = 100mm
08171-007
Figure 7. Maximum Allowable Current
for Various Current-to-ADuM4160 Spacings
As shown, the ADuM4160 is 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
noted, a 0.5 kA current would need to be placed 5 mm away
from the ADuM4160 to affect the operation of the component.
Note that at combinations of strong magnetic field and high
frequency, any loops formed by printed circuit board traces can
induce error voltages sufficiently large enough to trigger the
thresholds of succeeding circuitry. Take care in the layout of
such traces to avoid this possibility.
INSULATION LIFETIME
All insulation structures eventually break down when subjected
to voltage stress over a sufficiently long period. The rate of
insulation degradation is dependent on the characteristics of the
voltage waveform applied across the insulation. In addition to
the testing performed by the regulatory agencies, Analog
Devices carries out an extensive set of evaluations to determine
the lifetime of the insulation structure within the ADuM4160.
Analog Devices performs accelerated life testing using voltage
levels higher than the rated continuous working voltage. Accele-
ration factors for several operating conditions are determined.
These factors allow calculation of the time to failure at the actual
working voltage. The values shown in Table 8 summarize the
peak voltage for 50 years of service life for a bipolar ac operating
condition, and the maximum CSA/VDE approved working
voltages. In many cases, the approved working voltage is higher
than 50-year service life voltage. Operation at these high work-
ing voltages can lead to shortened insulation life in some cases.