separate grounds

Tie them together unless this creates a undesirable ground loop in combination with the external circuitry. Not knowing what the design decisions were that led to the initial separation, its difficult for us to guess what will happen if they are tied together.

The 1 Meg resistor may not do you any good depending on the impedance behind the voltage differential between each circuit.

What sorts of max ground voltage differences and energies are we talking about here? There are optoisolaters rated at many KV.

Why are they isolated from each other in the first place? What else is connected to the boards? What safety concerns are there?

If the analog is separated from the digital by the isolation specified then there is no requirement to join them.

However, if the magnetic isolator carries high dv/dt on it's windings, it may produce a high frequency differential between the two sections that can try to pump current through capacitances of the optocoupler, or through external tracking and the common safety earth plane.

Allowing for a capacitive return between the two grounds locally can provide a return path, if the optocoupler is affecred by this differential. Laying out a location for a couple of hundred pF of isolation-rated capacitance is a prudent measure in this case.

RL

If the circuits come together at an AD or DA converter, that is where you would like to place your single point ground, as part of the trace underneath the converter. But since you are using optoisolators, it would probably be ok to leave them completely separate. I like the 1 Mohm resistor idea, to keep them in relatively the same level.

Like a lot of other design items it depends upon the context. Is the statement: except that the two grounds could be at a voltage larger than that allowed by the optoisolators. true or hypothetical? What happens when the grounds are tied together? Enormous currents flow? The Big Bang is recreated? Leaving chunks of copper floating around is generally a bad idea, but sometimes necessary. If you don't join the grounds, or even if you do, how much pickup can you expect; radio stations, transformer currents, motor currents, etc.... ?

Ray