Hi David (and the group),
Take a look at the Technical Tidbits section of my site at
http://emcesd.com where you will find some experimental evidence for
your viewpoint. This month's article (at the bottom of the main page)
presents some data and links to two other articles.
Doug
On Friday, Oct 11, 2002, at 12:53 US/Pacific, David Heald wrote:
All,
I'm trying to convince a few people here that completely separating
the
digital and chassis grounding on our product is not always the best
way to
go. Unfortunately, a lot of the people I'm dealing with are ex
Bellcore
engineers who worked a lot with isolated grounds and are convinced that
isolated grounds are the only way to go. Now we're dealing with
optical
interfaces and speeds well in excess of 100MHz, so I really want to
see the
grounds tied together as much as possible.
While I know that combining the digital and chassis grounds is for the
most
part better once you get above a few hundred MHz, putting together
concrete
arguments is proving to be a bit elusive. I luckily have some high
level
backing that will let me push my views, but I am one person up against
a
team of industry vets.
If anyone has been in this boat before and won, could you share some
of the
tactics or arguments that you used? I know this issue has been
discussed in
the past, but a fresh discussion of the relative benefits of isolating
the D
and Cgnds would probably be beneficial to the group as well. See
below for
my views on the issue.
Thanks!!!!
Dave
My views for telecom equipment with a backplane and plug in circuit
packs
(and a good tight chassis around it all):
(Note that Analog grounds are outside of the scope of this statement -
I'm
focusing on Digital grounds and Chassis ground)
The benefits of separating Dgnd and Cgnd have to do with defining your
signal impedances and SI in general. When you place this system
inside a
Cgnd "balloon", all should be well but maybe there is some extra noise
due
to RF being trapped within the balloon.
However, if the Cgnd and Dgnd are tied together throughout the system,
the
effect should be similar to "heat shrinking" your conductive chassis
Cgnd
ballon onto your Dgnd. The single ended signal return currents should
still
follow their original paths and things should essentially remain
unchanged.
I could see some possibility (I'll avoid use of the word potential
here :o)
) for RF currents on the circuit pack card grounds due to RF fields
contained within the faraday cage, but I think these could be
mitigated by
clever bonding of the grounds on circuit packs. I think that
isolating the
faceplate from the Dgnd on the circuit packs but stitching the bottom
edge
(faceplate to backplane) Cgnd ESD guard band to Dgnd could alleviate
stray
currents on the cards and keep them relatively clean - all while still
maintaining the bonding of the Cgnd and Dgnd on a system level. The
idea
(as my brain developed it) is to keep the stray currents at the
periphery of
the card by limiting the through connections on the circuit packs and
forcing stray currents to flow near the edge of the card. The
backplane
should for the most part have Dgnd and Cgnd be one and the same.
Does this raise any red flags for anyone? I'm expecting at least a
few, but
this is the best scheme that I can come up with right now.
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