EMC Question

Dec 19, 2008 2 Replies
G

Hi,



I'm working on a design which is supplied from an external DC power source. I'm routing the supply (+ and -)via a common mode choke, so there's conceptually a separate DC ground either side of the choke (at least from a CM a.c. perspective).



The board also has a few RS232 ports and digital I/O lines, (each of which is fed through 3 terminal capacitors). These 'external' IO lines are presented on a connector.



My feeling is that any DC ground that I should make available on the external connector should be taken from *before* the common mode choke, since if it was the 'filtered' DC ground that was made available externally, the filter itself would get shunted if 'external' grounds were commoned.



_________ DC in + | | DC in + (Filtered)



----- | CM |----- DC in - |Choke | DC in - (Filtered)



----- | |----- | _________ | | _| Ground connection to ext.connector


I'm curious to know what the perceived wisdom is in this case? - Does this sound reasonable?



Many thanks



David


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D

Un bel giorno snipped-for-privacy@dfussell.co.uk digitò:

I suppose it depends on the application. In very noisy environments you probably need differential signaling anyway, therefore you can use the external ground, just to make sure that the common voltage between two different systems won't be too high.

In other cases, when you need a clean ground (i.e. single ended analog inputs) perhaps it's better to use a ground filter for each input, and connect each one to the relevant "internal ground".

In my experience though, ground splitting (i.e. analog vs. digital ground) is necessary only for very precise applications. Normally an uniform ground plane and a correct placement/routing works well, or even better.

emboliaschizoide.splinder.com
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J

yeah you might have to put the RS232 signals through the same choke or use some other some sort of isolation.

what is the choke here for?

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