RESET input. Do I really have to tie it to a processor-pin or can I use a
simple RC-circuit to reset it on power-up.
The RC works fine as a POR if you have a fast rise time but it is not good at
detecting glitches or brown out conditions which would also require a reset. As
you will not have a uC IO line to the RESET of the AD9851, there’s a
possibility you will lose communication to the AD9851 during a brown out or
power supply glitch, and this will require you to cycle the power supply –
maybe this is not a problem in a “one off “ demo system. But for the “belts and
braces” approach , go for a reset generator or an IO line.