Can I leave SDO open, or is there any reason I must connect a pull-up to SDO?
The AD5290 SDO , is a Serial Data Output Pin. it has an Internal N-Ch FET with
open-drain output. Even thought you do not use it. it can be left open.... but
good practise, i have done in the past is to tie it off with a pull-up to avoid
noise and high impedance feedback. just in case. Anyway having a resistor (e.g.
0605 or smaller) option designed in. you can pop and depopulate this at will
later in the design as you see fit.