Common Mode Chokes and Grounding

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Hi,

I am looking for opinions on grounding techniques, particularly in
automotive circuits.  I have designed a circuit that uses both digital and
analogue circuits and I was planning to connect these grounds at a single
point to attempt to keep digital noise away form the analogue section.  I am
also using a switch mode power supply operating at 300KHz so I was thinking
of using a common mode choke on the input to help EMC.  I was wondering the
best way to connect this, I am a bit confused......

I connect positive and ground to one side of the choke so on the other side
I will have a filtered positive and a filtered ground.  Does this then give
me a 3rd ground?  I was thinking about connecting the unfiltered ground to
the Chassis and running a ring around the outside of my PCB on both layers.
I was then going to connect the digital ground the filtered output after the
choke and connect the analogue ground like described above Some circuits I
have seen appear to have the ground connected together before and after the
choke so I cant see what the pint is of filtering the ground in the first
place in that example..

I hope I have been clear on what it is that I am trying to do and really
appreciate any feedback

Best Regards

AJ



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