To get this AC audio signal into the digital domain of an SOC I see two solutions. I'd like to know people's opinions or insights on how to decide between these solutions. Or offer a new idea.

Given these two circuits that do not share a common ground.
1) An audio signal in a 12VAC circuit (this is pre-existing to this project)
2) An SOC in a 3.3VDC circuit (this is the project being developed)

Option 1: Condition the AC audio signal to an AC signal with DC bias on the 3.3VDC circuit. Then use a single ended ADC on the 3.3VDC circuit.

Option 2: Use an external fully differential ADC.

To further complicate this the SOC is running Linux. It already has an audio subsystem that uses single sided ADCs. Using this would likely make the software easier because it would be like audio input to Linux on desktop systems, and buffering is already handled by the subsystem.

Because the the SOC is running Linux I suspect that it would be difficult to read an external fully differential ADC at a reliable rate from software. Thus an external IC would probably be necessary to buffer the data from the fully differential ADC. Unless someone knows a way to do this without one.

Any thoughts you might have on this are greatly appreciated.

 

Given these two circuits that do not share a common ground.

Why do you have separate grounds? Can't they be connected together?
If not you could use an opto-isolator such as this circuit.

To further complicate this the SOC is running Linux. It already has an audio subsystem that uses single sided ADCs.

Is there any reason you can't use those for the audio signal?

 

Thanks crutschow

Why do you have separate grounds? Can't they be connected together?

I don't think so. The AC audio signal needs to be acquired at the end of a run of wires from the AC circuit. The DC SOC circuit is powered by a DC adapter from standard 220V electrical wiring.

Is there any reason you can't use those for the audio signal?

Yes, this is "option 1". To condition the AC audio signal for acquisition with these single sided ADCs. I just don't know if there are other issues at play. Maybe this is super easy and cheap to condition the audio signal in this way, and maybe external fully differential ADCs are super hard and expensive. In that case the choice would be clear. Or perhaps the complexity and expense is the other way around.

Thanks for the link! I'll check that out.

 

Would transformer coupling suffice? An audio coupling transformer such as this, this or this?

 

The AC audio signal needs to be acquired at the end of a run of wires from the AC circuit.

Don't understand what that means.
What is "the end of a run of wires"?

 

Would transformer coupling suffice? An audio coupling transformer such as this, this or this?

Ah, I suppose it might. I see that these are used specifically to isolate audio circuits, among other things. Thanks for this idea.

 

To get this AC audio signal into the digital domain of an SOC I see two solutions.

Solution 3: transformers.

 

Don't understand what that means.
What is "the end of a run of wires"?

The SOC DC circuit is intended to be installed in locations where there are some wires from an AC circuit coming out of a wall. If all one has access to is the common and signal wire from the AC circuit then I think one can not power the the SOC circuit from the AC circuit. Other than using the AC circuit to power the SOC circuit I do not know how to connect the grounds of the two circuits.

Is this what you meant about connecting the grounds? To use a rectifier to power the SOC circuit from the AC circuit that is producing the signal?

I'm not an EE, I just did some research to try to understand this problem myself. So if I'm way off here please let me know.

 

I still don't know what is the problem you have with the grounds.

Stereo components are connected together through cables with RCA jacks on them. One of the two wires on the RCA connector IS the ground, and when you plug it in to the two components, the grounds are connected together. What is different about your circuit?

Now, sometimes, when the two stereo components are plugged into different line circuits, you get what is called a grouind loop, which is where the two grounds are at slightly different potentials and you get a hum because current flows through the ground connection of the audio cable.

But you said the SOC device is powered by a DC adapator, which probably means it is floating (has no ground of it's own). If this is the case, connecting the audio cable will connect the grounds with no possiblilty of a ground loop.

Bob