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EMC Question

Discussion in 'Electronic Design' started by [email protected], Dec 19, 2008.

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  1. Guest

    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
     
  2. dalai lamah

    dalai lamah Guest

    Un bel giorno 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.
     
  3. Eeyore

    Eeyore Guest

    Ditto. The system as a whole must be considered.

    Graham
     
  4. Jasen Betts

    Jasen Betts Guest

    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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