Designing a DC stage for a mixed signal board
This is a very comprehensive question and answering it within one post may be difficult. Since I cannot comment yet I guess I'll make a full post of the advice I have.

Keep in mind what the loads consuming by each of your supplies will be. If the loads are high enough consider using buck converters instead. Unless you're not too concerned about power efficiency which could be the case.

Definitely separate your analog and digital grounds if your analog is very sensitive to noise (i.e. high speed I/Os). However, if the frequencies of operation are not that high you may not be impacted by noise all that much from a power supply perspective.

It's generally not a good idea to nest power conversion stages (i.e. a 12V to 5V stage followed by a 5V to 3.3V stage). This could be neglected if using straight LVRs/LDOs since efficiency is only Vin/Vout. But again check your load demands since DC/DC converters can be more efficient.

For your question &quot;Should I place the DC stage on one small area on the board or put the regulators as close as I can to their relevant consumers?&quot; This falls in the realm of power delivery networks (PDNs) and it's very application specific. If you will have high dI/dt events (i.e. high load steps) then if you're too far away from the consumer, you'll be hitting parasitic inductances that will compromise the effectiveness of the regulator. Also, the longer the copper trace the more resistance you'll have.


I won't make this post longer unless you want to hear more. But for now these are the recommendations I'd give you.