Common Mode Chokes and Grounding


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


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