Data Sheet

AD7401

Rev. F | Page 17 of 18

APPLICATIONS INFORMATION

GROUNDING AND LAYOUT

Supply decoupling with a value of 100 nF is strongly recom-

involving high common-mode transients, care must be taken to

ensure that board coupling across the isolation barrier is mini-

mized. Furthermore, the board layout must be designed so any

coupling that occurs equally affects all pins on a given component

side. Failure to ensure this may cause voltage differentials between

pins to exceed the absolute maximum ratings of the device, thereby

leading to latch-up or permanent damage. Any decoupling used

must be placed as close to the supply pins as possible.

Series resistance in the analog inputs must be minimized to

avoid any distortion effects, especially at high temperatures. If

possible, equalize the source impedance on each analog input to

minimize offset. Beware of mismatch and thermocouple effects

on the analog input PCB tracks to reduce offset drift.

EVALUATING THE AD7401 PERFORMANCE

A simple standalone AD7401 evaluation board is available with

split ground planes and a board split beneath the AD7401 package

to ensure isolation. This board allows access to each pin on the

device for evaluation purposes. External supplies and all other

circuitry (such as a digital filter) must be provided by the user.

INSULATION LIFETIME

All insulation structures, subjected to sufficient time and/or

voltage, are vulnerable to breakdown. In addition to the testing

performed by the regulatory agencies, Analog Devices has carried

out an extensive set of evaluations to determine the lifetime of

the insulation structure within the AD7401.

These tests subject populations of devices to continuous cross

isolation voltages. To accelerate the occurrence of failures, the

selected test voltages are values exceeding those of normal use.

The time to failure values of these units are recorded and used

to calculate acceleration factors. These factors are then used to

calculate the time to failure under normal operating conditions.

The values shown in Table 7 are the lesser of the following

two values:

•

The value that ensures at least a 50-year lifetime of

continuous use

•

The maximum CSA/VDE approved working voltage

Note that the lifetime of the AD7401 varies according to the

waveform type imposed across the isolation barrier. The

i

Coupler insulation structure is stressed differently depending

on whether the waveform is bipolar ac, unipolar ac, or dc.

Figure 30, Figure 31, and Figure 32 illustrate the different

isolation voltage waveforms.

0V

RATED PEAK VOLTAGE

Figure 30. Bipolar AC Waveform

0V

RATED PEAK VOLTAGE

Figure 31. Unipolar AC Waveform

0V

RATED PEAK VOLTAGE

Figure 32. DC Waveform