I would like to confirm about unused LVDS input pin of ADN4668.
What is the recommended state? (Open or termination?)
To avoid chatter on the output, leaving the input open is best. The ADN4668 has open circuit failsafe, so the output state will stay in logic 1 state.
By contrast, adding termination to the input but not driving it, will result in an indeterminate state for the output (possible chatter on the unused channel) as the ADN4668 does not have failsafe for this mode and has a symmetrical receiver threshold (no offset to receive low differential input voltage as logic 1).