We occasionally experience a failure of the AD5420 where the output
will shift up tens to hundreds of milliAmps (or more). This failure
occurs after our product is Hi-Pot tested. Our Hi-Pot test is one
minute of 500 VAC between the analog output and the chassis connection
and 1500 VAC between the power lines and chassis connection. Our product is powered through a transformer.
For example, one AD5420 was calibrated to a normal 4-20mA
output. After Hi-Pot testing, the data-word that produces a 4 mA
output, now produces a 25 mA output. Likewise the data-word that
normally produces a 20 mA output produces a 134 mA output. This is a
permanent condition after Hi-Pot: regardless of the number of power
cycles, the abnormal 25mA to 134mA output swing remains.
I have verified that there is no difference in the serial bus signal
levels, timing or data/control register values before and after this
Hi-pot induced condition appears.
Note that I have discovered a brute force way of correcting the issue:
I briefly touched the R3sense output (TSSOP Pin 18) to ground. This
causes AVDD to collapse and the product microprocessor to reset.
Upon resetting, to output signal swing was restored to the normal 4mA to
I've applied this brute force fix to 5 different defective units, and
have cleared up the problem each time.