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Tone (audio) generator : breaking the ground loop
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ratatax:
Hi,
It's me again with my ground problems.
I'm designing a sound generator in a grounded/earthed metal enclosure... Everything was fine until I had my first ground issues, when connecting my device to other mains earthed devices.

The easy solution would be to get rid of earth, but I can't disconnect it from my chassis since most AC adapters have (harmless but nasty) leakage, like 20V creating a buzz feeling when touching the chassis.

So I tried to separate grounds by using an isolated DC/DC converter for my analog/output circuitry. However a problem shows up where the DAC send its signal to the opamp. They both have different ground references (see schematic below).
I initially thought the DC blocking capacitor at the DAC output was enough... But I feel it's not. It gets rid of the absolute difference between grounds, but coudn't the DAC be damaged if the potential between the two grounds changes rapidly ?

I'm searching for the proper solution to this problem

NiHaoMike:
Move the DAC to the other side and use a digital isolator.
ratatax:
Possible but costs quite a lot : needs another isolated DC/DC converter (for +3.3V) and a several digital isolator chips that are quite expensive.
Is there a simpler way ?
MagicSmoker:
No, your scheme won't work because there is no defined return path for current from the DAC (except for parasitic capacitances).

Just replace the RC network with a digital isolator to transmit the DAC information digitally. You won't need an additional isolated power supply or much in the way of expense. Silicon Labs makes a whole range of digital isolators for tasks like this and which don't cost too much - maybe $1-$3.

SiliconWizard:

--- Quote from: ratatax on November 16, 2019, 04:02:12 pm ---Possible but costs quite a lot : needs another isolated DC/DC converter (for +3.3V) and a several digital isolator chips that are quite expensive.

--- End quote ---

I would also suggest isolating at the digital level.
Why would you need another DC/DC converter? Can you not generate the +3.3V voltage from the existing isolated DC/DC converter with just an additional LDO?

The digital isolator ICs are not that expensive (define your budget). You can find them as 4 isolated I/Os. If your DAC is SPI, that should do it?
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