Data Sheet AD7405
GROUNDING AND LAYOUT
It is recommended to decouple the V
DD1
supply with a 10 µF
capacitor in parallel with a 1 nF capacitor to GND
1
. Decouple
Pin 1 and Pin 7 individually. Decouple the V
DD2
supply with a
100 nF value to GND
2
. In applications involving high common-
mode transients, minimize board coupling across the isolation
barrier. Furthermore, design the board layout so that any
coupling that occurs equally affects all pins on a given
component side. Failure to ensure equal coupling can cause
voltage differentials between pins to exceed the absolute
maximum ratings of the device, thereby leading to latch-up or
permanent damage. Place any decoupling used as close to the
supply pins as possible.
Minimize series resistance in the analog inputs to avoid any
distortion effects, especially at high temperatures. If possible,
equalize the source impedance on each analog input to minimize
offset. To reduce offset drift, check for mismatch and thermocouple
effects on the analog input printed circuit board (PCB) tracks.
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 AD7405.
Analog Devices performs accelerated life testing using voltage
levels higher than the rated continuous working voltage.
Acceleration 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 20 years of service life for a
bipolar, ac operating condition and the maximum VDE
approved working voltages.
These tests subjected the AD7405 to continuous cross isolation
voltages. To accelerate the occurrence of failures, the selected
test voltages were values exceeding those of normal use. The
time to failure values of these units were recorded and used to
calculate the acceleration factors. These factors were then used
to calculate the time to failure under the normal operating
conditions. The values shown in Table 8 are the lesser of the
following two values:
• The value that ensures at least a 20-year lifetime of
continuous use.
• The maximum VDE approved working voltage.
Note that the lifetime of the AD7405 varies according to the
waveform type imposed across the isolation barrier. The
iCoupler insulation structure is stressed differently, depending
on whether the waveform is bipolar ac, unipolar ac, or dc.
Figure 34, Figure 35, and Figure 36 illustrate the different
isolation voltage waveforms.
F
igure 34. Bipolar AC Waveform, 50 Hz or 60 Hz
F
igure 35. Unipolar AC Waveform, 50 Hz or 60 Hz
F
igure 36. DC Waveform
0V
RATED PEAK VOLTAGE
12536-034
0V
RATED PEAK VOLTAGE
12536-035
0V
RATED PEAK VOLTAGE
12536-036
Rev. A | Page 19 of 20
