Option 1 : one single groundplane
In my opinion, the best option. Just keep the analogue and digital traces as far apart as possible
Option 2 : two, joined by a ferrite bead somewhere
Bad plan for digital signals. The I2S will lose its sharp edges and you will likely see jitter as the sample point moves. Your circuit should be designed to handle up to the 5th harmonic of a signal with minimal losses to get a good edge to sample. This rules out a ferrite connection.
Option 3 : two, joined by a trace under those resistors carrying the I2S signal
OK option, but power supplies should be isolated for each board, or the power connections should also be made at or around the same point. The reason for this is that both ground and power can carry a high frequency signal. It doesnt care which plane it jumps to as it is ac
Option 4 : two, joined at the power supply
Bad idea, the return path will be a big ground loop that will run round the gap and transmit noise onto the entire board via the power supply. Also, very poor emc and esd protection as the high frequency created on one plane may just jump across the planes using the I2S wires and fry the chips
Option 5 : two, with the I2S passing through some isolating device such as the ISO150 (costly and big)
Good option, I suggest two boards are used with completely isolated power supplies or a single board with three islands and one psu for each. The ISO150 sits as in the middle island and only allows the I2S to hop.
Option 6 : two, joined by a pair of schottky's, back to back
Bad idea, there is no low voltage DC return path. Noise will be created at a level equal to the voltage drop across the diode on each board. You will most certainly hear this on the output
Option 7 : two, joined by a resistor
Similar to option 6, we're looking to lower impedance, not create it
What you have to remember is this - any signal will take the path of least impedance. Note that two ground planes near to each other form a capacitor, a length of track is a resistor (and inductor) and a via is a bigger inductor and little resistor. A signal will take the easiest path, wherever it has to jump. This will depend on frequency.
The best board is a 4 layer board with central ground and power planes. This allows return current to flow beneath any signal thus giving the smallest possible loop area. By minimising loop area, we minimise coupling between signal lines. By keeping a distance, we prevent crosstalk at the signal decreases proportional to a power of distance from the track.
I suggest the following
1 - Use a 4 layer board (or 5 layer for two supplies, alternatively split only the power plane (2nd layer from base, make the ground the third from base and run all important signals on the top layer thus next to a complete plane)
2 - Keep power and ground on central planes
3 - Decouple the supplies next to each chip
4 - Keep Analogue and Digital as far away as possible
5 - Ensure separation between signal lines
6 - Match the length and impedance of the I2S lines for the best signal quality
7 - Place in a metal box, but only provide a single starpoint to the box for all grounds to avoid loops forming
