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EMC Question

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
 
D

dalai lamah

Jan 1, 1970
0
Un bel giorno [email protected] digitò:
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.

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.
 
J

Jasen Betts

Jan 1, 1970
0
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.

yeah you might have to put the RS232 signals through the same choke
or use some other some sort of isolation.
_________
DC in + | | DC in + (Filtered)
----- | CM |-----
DC in - |Choke | DC in - (Filtered)
----- | |-----
| _________
|
|
_| Ground connection to ext.connector

what is the choke here for?
 
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