I2S ground?

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I'm looking to transfer I2S between PCBs along a short ribbon cable. I have read that the cable should include alternating ground / signal.

But everything I have seen (schematics etc) there is no ground involved with I2S, just source to sink.

So how does the grounding fit in:

- Are the ground lines connected only at the source end and left floating at the sink end? (more like a shield?)

- Or, should they connect the signal pin on the receiving IC to the ground plane on the source chip's PCB?

- Or, should the two ground planes be connected along the ribbon? NB: I have 2 separate ground planes, as the source chip is USB powered.
There could be issues.

Grounding is not straightforward, but one thing is always true:-

every signal requires a ground return.

Generally you want to keep the ground returns for different signals separated, but not always; it's not always practicable.

We don't generally like more than one connection between boards, this can result in a ground loop, but...

...it's not uncommon for every second conductor in a ribbon cable to be ground. This can improve crosstalk between signals and EMC. In general though, a single connection between boards is easiest to debug. It's not possible to give hard-and-fast rules that will enable a beginner to implement a satisfactory grounding scheme, but start with a single connection to each board.

Google 'star ground', but also have a look at 'multipoint ground'.
It is usually good practice to have a signal wire, ground, signal wire and so on on a cable. As stated above, signals always have a return current as a result of kirchoffs laws. Even though the receiver might be of high impedance, there is always capacitance and current will flow.

Now, if you have a high speed signal, think about the current that goes to the receiver and the current that comes back through ground: it will make a loop which again induces magnetic field. This adds inductance to the circuit (slows down signaling speed) and causes EMI radiation. This is why high speed signals are usually differential. If every second wire is ground, then there is an "easy" way for signals to propagate; return current flows close to the input current and inductance is minimized (because of a small loop) and the best thing is that with solid ground plane and multiple ground wires the current will find the easiest path to flow. This is because of the inductance the loops create.

So yes, ground planes should usually be connected together. It is a matter of optimizing wire count and EMI radiation and signaling speed. Probably your I2S circuit will work with one ground wire for multiple signal wires but grounds must be connected.
I personally don't see a problem in USB power or using ribbon cable for power delivery but it depends on other constraint whether they are good or not. I wouldn't use USB voltage directly to drive at least sensitive analog electronics. USB has nominal voltage of 5V but it may fluctuate and there is no guarantee for ripple etc. If it is properly post-regulated and filtered, then yes. I would use it. The same applies to ribbon cable as well.
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