wire from Digital GND to Analog GND reduce noice

[davyzhu]

Hello all,

I was told that when connecting analog chip to digital chip, we cannot put Analog GND and Digital GND on one plane, they should be connected by one wire. But why this technique reduce noice from Digital GND to Analog GND? If I connected them with two wire, what will happen?

Regards,

DAVY

 

[IanP]

This single connection works like a small inductor which creates a barier for HF noises. If you connect AGND and DGND in more places this may create ground loops and grounds will not be separated...

This topic was discussed in this forum some time ago.. so for more details just search the forum..

 

[ee_wmkab]

Digital signal consist a lot of high frequency transition which result in a lot of the unwanted high frequency signal in the signal line/ground line. This unwant high frequency signal will act as noise to the analog circuit if they go into the analog circuit.

All the signal need a "return" path for close loop current flow, and the "return" signal will choose the lowest impedence path to go back to the ground(reference ponit). If the digital ground plan is connected to analog ground plane at many point, those connection point may be act as a low impedence path and those return signal will go into the analog ground plane which introduce interference to the sensitive analog circuit. If we only provide one single connection point between analog and digital ground AND a propoer layout design to ensure all the return signal won't go into analog ground at other connection, the digital noise that go into the analog plane will be greatly reduced.

 

[Circuit_seller]

Hi

If we have a track which goes from digital part to analo part, then what is the return path.
the return signal should go from the analog part and the one-connection between DGND and AGND to back to the origin, so this signal introduce a lot of noise on the analog part, so i think i t is better to take AGND and DGND the same.

Regards

 

[brmadhukar]

The digital circuits introduce a large demand for high frequency power exchange and this inturn make the DGND noisy. A thin wire maintains the DC potential between AGND and DGND and also because of inductive action prevents HF noise from entering the AGND.

However on many ICs that have both DGND and AGND you need to connect them together using a track. They are not be connected to different grounds.

Hope this helps,
B R Madhukar

 

[eziggurat]

Try using the stitching technique described in www.compliance-club.com. In issue 54 of the journal, the author for the PCB article describe the stitching method improves signal quality.