include Schmitt trigger on clock or data lines?
We strongly advise that you use Schmitt triggers when interfacing the AD420 to
opto-couplers. We have done extensive testing with the AD420 and found that
only opto-couplers with very fast transition times (e.g. HPCL3531)
optocouplers will reliably drive the AD420 digital interface with. These have
a slew rate of 1000V/us and are relatively expensive.
There are no Schmitt triggers on either the clock or the data inputs of the
AD421 so you need a very clean (sharp transitions) input signal. If you use
slow optocouplers then it is very likely that the AD421 will see multiple clock
transitions and you will get framing errors. Due to inherent noise in the
digital signals, you really need some hysteresis in there for reliable
operation to prevent false triggering.