For isolated power supplies the capacitor may be used to manage common-mode
noise associated with the switching components; e.g. MOSFET, rectifiers, and
transformer. This may be more helpful when using primary side control and
isolated feedback.
Bert Simonovich's description is the only other usage I have routinely seen and
would consider reasonable.
However, some designers insist on leaving "hooks" in designs to manage the
possibility of noise and ground loop problems when dealing with digital and
analog domains. Sometimes the only thing they are willing to listen to is to
make the parts "do not stuff" on the initial build. That encouragement often
follows a religious discourse on why a single ground plane with managed
placement of components and traces should be the preferred implementation for
all but the most extreme noise sensitive designs.
Chris Belting
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On ;
Behalf Of Loyer, Jeff
Sent: Monday, November 14, 2016 3:00 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Caps between "isolated grounds"?
I was looking at some designs and found different ground symbols connected by
capacitors. Can anyone explain why this might be done? Everything I've seen
or heard says this is a bad thing (I would connect them directly), but I want
to be sure I'm not missing something. I think there are some A/D devices which
have specific guidelines for separating digital and analog grounds, but I don't
think they'd be connected by caps.
Thanks,
Jeff Loyer
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