The used ADC has tristate outputs. Is it neccessary to use pull up or down
resistors at AduM1402 inputs ? And if not how is the behaivior of the AduM1402
output, if the ADC output is in high Z state ?
Driving high impedance inputs with a high Z output will give unpredictable
results. Pull-up or pull down resistors should be used to guarantee a
particular state for the coupler inputs when the input is basically floating.
I can think of a couple of undesirable behaviors that could result. If stray
noise pushes the input across the threshold you could build up oscillations
that would cause high or variable supply currents even if the output of the
coupler was disabled. If the output was active it could be a source of
asynchronous noise in the system.
The long and short of it, is that the inputs of the coupler should be in a
known state when the ADC outputs are disabled. This will require something
like a weak pull-up or pull-down resistor.