Various industry specifications call out separate "circuit"
and "chassis" (sometimes called "frame" or "safety")
grounds, along with voltage isolation, bleed resistors,
bridging capacitance etc. requirements. Some examples
include IEEE 1101.1-1998 (Base 3U/6U/9U), VITA1.1-1997r2003
(VME), ANSI/VITA 46.0-2007 (Base VPX), PICMG 2.0r3 (Base
CPCI), IEEE 802.3 (Ethernet), EIA-644-A (LVDS), etc. I
understand there are similar specifications for some types
of consumer products. Also, while not part of specification
sometimes designers (or app notes they work from) will call
out separate "digital" and "analog" grounds, connected at
single point with zero ohm resistor or ferrite bead. While
there may be use case for such topology, it frequently
causes problems. Regards, Ivor
On 11/14/2016 2:00 PM, Loyer, Jeff (Redacted sender jwloyer
for DMARC) wrote:
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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