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Proper ground connection of two pcb

postip , 07-25-2021, 08:32 AM

I have two seperated pcb one is power supply pcb and the other one is noise sensitive DAC pcb.

I put vias on both pcb to make a pin to pin power connection please see attached picture.

In power pcb i use noisy isolated dc/dc converter (switch freq. 100khz). To clean up noise, i use LC filter + cap multiplier right after isolation and then a linear regulator.

I must connect power supply ground plane to the DAC pcb ground plane. The question is, which location/locations is good to do that on both pcb?

Thanks in advance...
robertferanec , 07-26-2021, 01:52 AM
I am not expert for sensitive circuits, however I am not exactly sure what does it mean "noisy ground plane" - actually I think I know where this is coming from, I am just not sure if that is how it works. After doing a lot of videos with Eric and Rick, I would say, understanding return currents could be the key in this case.

If the power supply board (the first one) is designed properly, I am not sure if we can say it's ground plane is going to be noisy. If the whole GND plane would be noisy, there may be something wrong with the design. In my opinion, all the "noise" should be local in the critical places of components and layout. I do not think it should spread through the whole GND plane .... if it does, it would mean the return current path is not controlled properly .... but I may be wrong.

I guess, this is not really answering your question, but I hope it will help to have a look at this system through a different point of view - how the return currents will flow.

These videos may help:
What Every PCB Designer Should Know - Return Current Path (with Eric Bogatin) https://youtu.be/icRzEZF3eZo
Ground in PCB Layout - Separate or Not Separate? (with Rick Hartley) https://youtu.be/vALt6Sd9vlY
qdrives , 07-26-2021, 08:15 AM
For the linear regulator pick the one with a good PSRR value.
And video from Steven Sandler: https://passive-components.eu/addres...icotest-video/
anovickis , 08-15-2021, 09:45 PM
It all depends on how much noise is tolerable

For example if you have DCDC switchers on the first board, they will radiate from the transformer/coil in the near field and that will couple in on your other board if it is in proximity
On the other hand, 4-5 inches apart you aren't likely to see any of that - but you are probably running some digital lines over to the other board also

If you are using 12-14 bit adc/dac's you might get a bit of noise in the first case but that's not bad. If it's a 24 bit system - then you have take careful steps to isolate with LDO's and common mode choke before that

It also, of course, depends on the amplitude of your analog signals - Are you looking at things that are a few millivolts, or 2 volts p-p - that makes a big difference

Keep track of your current paths, and isolate analog from digital traces - and you should be good

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