Hi AdvaRes,
there isn't short answer for that.
you will need to know the power supply parameters like:
RMS noise, ptp noise, switched or LDO, is switched at what frequency and what is the output capacitor and Inductor.
if it's LDO then it is more simple, for example if you will use LDO of Linear LT1764 then you should had Sin of 10mV. but don't forget the RMS noise!
if it is Switched then you will need to calculate the output noise ripple using the power supply datasheet formulas. for example as LTC3611 page 14.
Ran