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Analog ground and Digital ground

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joersam24:
Hi to all,
I just want to ask if it is really a requirement to connect a digital ground to analog ground? It is possible not to connect the two to completely isolate the digital part to analog part? Thanks...

IanB:
Let's pretend all circuits require two wires (out and back), and that you connect only a single wire from the analog part to the digital part. Will the circuit still work?

Of course you could connect no wires at all and use a transformer or opto-isolator to connect the two parts of the circuit, but now you might need to have two separately isolated power supplies...

amspire:
If the analog circuit does not need to share a common supply line with the digital circuit, then of course the two grounds can be isolated.

Often they need to share the same 0V, and in that case, it is usually common to make sure the two grounds only connect at one point - you do not want any ground loops between the analog ground and the digital ground.

But I suspect that you have a particular case in mind, and you might have to explain the situation. Are both the analog circuits and digital circuits going to connect to mains earth externally or to other  earthed equipment?

Richard.

joersam24:
I understand it already, thanks to you IanB and Richard..
By the way, let me share to you one problem I encountered in our project.. We have a PSTN telephone with interfce to a tablet PC project... Now when making a call and the tablet is powered only by a battery there is no problem.. However when the charger is connected to the tablet there is a noise that occurs in the line... We used an audio transformer between the telephone chip going to tablet PC. When I tried to connect the grounds of primary and secondary of audio transformers the noise caused by the charger disappeared.. Do you have any idea why this happened? Grounds of primay and secondary of audio transformer should not be connected isn't it?

IanB:
This is outside my experience (I don't know much about electronics), but think there might be an analogy here with mechanical systems.

If you had some piece of equipment that was jumping around and vibrating, you might reduce the vibrations by tying it down firmly to the ground with ropes or bolts. By analogy in a circuit you might reduce unwanted noise in the circuit by tying the circuit firmly to a ground reference.

But with the mechanical system, anchoring the equipment to the ground has reduced the isolation. If the ground vibrates (like with an earthquake), the equipment tied to the ground will get shaken around. Likewise with the circuit--if you tie it to a ground reference, it will be vulnerable to unwanted voltages that get transmitted through the ground path.

I think good circuit design involves balancing both effects while also paying attention to safety considerations.

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