Analog ground in mixed signal circuit using isolated DC-DC module?

[slow_rider]

I have a circuit which is composed mainly of MCU controlled CMOS switches. The input to the system is 5V which will feed the digital side of things. Tapping into the 5V rail is a dual output DC-DC module that I have around. The module taked 5V input and outputs +18V / COM / -18V. This common connection will be the analog ground reference for the analog part of the circuit.

My question is - the CMOS switches in the circuit are controlled from the digital side of the circuit so the analog and the digital grounds should be on the same voltage potential (0V) to avoid possibly nasty behavior. What would be the best way to make sure the analog and digital grounds are at the same potential without losing the isolation if at all possible.

Thanks!

[T3sl4co1l]

What are the circuit inputs and outputs referenced to, and is this just switching (signal mux??) or is there anything else (gain, filtering, ?) present?

For example, if they need to be isolated, obviously the digital switches need to be as well.  If they are all common ground, you're best off laying out the digital logic circuit on one side (so all the switching noise and supply ripple circulates in its own area), then routing traces from that region to the analog switches.  If the switches are supplied by +5 or +/-5 or 18 or whatever, filter whatever respective supplies are nearby (to whatever margin is required for signal quality).  Everything should be over ground plane (just as a matter of course), and no splits or slots are needed or desired.

Tim

[slow_rider]

The input and outputs of the circuit are referenced together to ground, the one of the ac power (wall). There is fairly little digital signaling going on and there is no safety issues here so I thought perhaps I would limit the slew on the digital signals and avoid isolating them Galvanically which adds more parts and costs $$$.

The common output on the output side of the dc-dc module is essentially floating, you say I should hook it up to the power ground using a single wire (star ground formation)?

[T3sl4co1l]

Well, it all has to be common ground, otherwise you have floating supplies...

What's the +/-18V for?

Star grounding is usually a dumb idea, for reasons unappreciated by those suggesting them.  Solid ground plane (or as best you can do on a 2-sided board, bonding top and bottom ground fill with frequent vias) is almost always much better.  The remaining microvolts can be handled with appropriate layout.

DC-DC converters are very dirty, so it's not so much that you need it referenced to ground (aside from DC), but that the supplies are filtered with respect to the best ground you have, and avoiding ground loop currents from the switching circuit.  A CLC filter (say, 10uF + 1uH + 10uF, with ceramic 0.1uF capacitors thrown in near each load) would probably be a reasonable start, but the particular values and approach do depend on just how noisy it is, and how sensitive your circuit and output are, and at what frequencies.

Tim

[slow_rider]

Thanks for the detailed answer tim.
The +/- supplies are for the CMOS switches, these need the supply to be a bit higher than the max. amplitude they can pass. The switching freq. of the DC-DC is over 70KHz and my signal is in the audio range so even if the switcher is dirty in the higher freq. range that is fine with me.

As for construction. I'm afraid I don't have the choice for a 2 layer PCB but rather a pref-board inside a metal box. That is the closets thing I could have to a ground plane. The problem is that the board with the traces and all will be at some distance from the box wall so won't act the same manner as a PCB with planes on both sides.