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Primary Ground and Secondary Ground of Audio Transformer

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joersam24:
Hi to all,

I just want to ask regarding the problem I encountered in our PSTN Telephone with interface to tablet PC project. This is the case, when the tablet PC is powered only by battery PSTN calling is okay. However, if the charger is connected to the tablet, there is a noise that occurs during telephone call. We used an audio transformer as an isolation between telephone chip and tablet PC. When I tried to connect the primary ground and secondary ground of audio transformer the noise caused by the charger disappeared..
Any idea why this happened? I don't think the noise was caused by ground loops because the two grounds are isolated by an audio transformer..

And one thing.. Primary ground and secondary ground of audio transformer should not be connected right?

IanB:
You already asked this question in this other thread. It's better to keep that thread going than start a new one.

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