Grounding practices in mixed signal multi board circuits

[lungdart]

I have a mixed signal multiboard circuit, and I am not sure on what the best way to design the grounding would be.

The first board is a 4 layer board, with a ground plane and power plane layer. Digital power is kept separate from the analog and there is a single ground bridge joining the two ground planes. All the mixed signal connections run over the ground bridge on this board. There are 2 board connectors for the sub circuit to connect with, sitting above the ground bridge, set up similarly (ground layer, power layer, ground bridge connection with all mixed signal traces running over it)

Now as you can see I have a ground loop between the bridge on the main circuit, and the bridge on the sub circuit. How do I avoid this ground loop, while still being able to run mixed signal lines across from the digital to the analog planes over a bridge? Would I have to run all signals back through the connector, and cross over a single bridge? That could make for a massive connector!


HELP!

[jahonen]

This has been discussed several times in the past. Given all the difficulties associated with this ground splitting, I would primarily consider making just one ground plane and placing components so that no digital/power currents travel through analog parts. That is often very satisfactory solution.

Are you sure that this option wouldn't work?

Regards,
Janne

[nctnico]

One big ground plane AND keeping noisy circuits away from sensitive circuits is the best solution. If you need to run sensitive signals over longer distances use differential signals. Lots of audio chips have differential inputs/outputs these days to allow them to be used in crammed mobile phones and tablets.

[JuKu]

There is no ground, there are only currents and return currents. Separate these, and you will be ok. Kirchhoff is your friend, telling you that on a solid ground plane, the return current runs under the signal current, so you only have to worry about signal tracks. Split the ground, and you have to worry about return currents as well. In other words: In most cases split ground, at best, does not hurt much.

[lungdart]

Thanks for your input!

So is it recommended to use a single plane for both boards? Or is it kosher to use a dedicated digital plane on the main board with the micro and higher frequency stuff, and just use the single analog plane on the daughter board?

[w2aew]

Here are two really great application notes that discuss grounding, decoupling, etc...'

http://www.analog.com/library/analogdialogue/archives/46-06/staying_well_grounded.html

and

http://www.analog.com/static/imported-files/application_notes/AN-202.pdf

[Neilm]

For further reading, go to http://compliance-club.com/ and search for the various series done by Keith Armstrong. He has done a few good series on layout of circuit boards and equipment