mixed digital/ADC analog ground plane question

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Four of the ADC bits in an AT90CAN128 are being used a digital outputs (they do not switch while any ADC conversion is in progress). The other four ADC bits are used for analog ADC inputs.

With a conventional analog ground plane covering all eight ADC pins (page 281 of the 7679C–CAN–01/07 data sheet), the four port F digital outputs would cross from the analog ground plane to the digital ground plane.

I'm thinking I could just run the digital ground plane all the way to the four digital pins and keep the analog ground plane isolated to the four analog pins, with a single common tie point between the digital and analog ground planes. I assume the internal port F uses a common internal ground (possibly referenced to pin 63?). The idea is to avoid potentially convoluted/noisy digital signal PC trace ground return paths through port F.

Does anyone have any recommendations on a good strategy for setting up mixed use PC board ground planes with some analog and some digital pins all on the same port like this?

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If you don´t switch the outputs and there is not much DC current in/out of the pins i see no big problem.

Also WHEN switching, then wait a little before starting the next conversion.

digital switching signals may influence the analog part when crossing then analog section.
DC currents in the ADC area may cause groung bounce and this causes offset currents in the ADC.

Klaus
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Look at: www.megausb.de (German)
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Thanks for the help. Your points are well taken.

Since good EMI/EMC PC design is sort of a black art, I was thinking about noise coupling through the digital I/O lines crossing over both ground planes and possible signal trace noise radiation from a convoluted ground return path that cannot follow a direct digital I/O ground return plane. My hope is not to do anything that might encourage any ground loops either. I know, I want too much :).