4 layer PCB, mixed signal, multiple different voltages

Question

For a mixed signal PCB with DACs, ADCs, MCUs and multiple analog opamp circuits. With +/-15V to power the opamps, 3.3V for all the digital stuff and a couple reference voltage rails (1.5V and 3V).

Can someone suggest a layer breakdown for a 4 Layer PCB? Obviously ground for one of the inner layers and I was also thinking in the digital part of the board I could make the second inner layer a 3.3v fill and then in the analog part I could leave it unfilled and run power traces (+/-15v and 1.5/3v ref) there. Is this a bad idea?

Edit: Forgot to mention components are on both sides of the PCB

Answer 1

Typically you'd have L2 as solid ground, and L3 would be the power layer which is made up of most if not all power rails in the design. Outer layers are to connect all your signals together but you can put fat power traces there too if necessary.

Answer 2

Do you need more than one layer for the signals (ignoring power and ground traces)?

For analog stuff like this I normally try to arrange all the chips and discrete bits such that it can be routed without vias on L1. Vias are always going to be a way to get noise into your signals. Consider ground guards around bits of circuit that are unrelated to others.

Then L2 solid ground.

Then L3 would be the vref normally as that tends to be like a ground plane in terms of number of connections and is also a way of generating cross talk.

Finally use L4 to bring all the other power rails to the chips. Note that you need good decoupling always, but especially important if delivering power to multiple chips with traces rather than planes.

But I am a software engineer, so others might have better ideas!