Hi Boris,
Say for example in our PCB if we have analog gnd, digital ground and
chassis ground and we are going to place this pcb in a chassis ( chassis
body forms chassis gnd).My understanding would be chassis acts as sink for
the noise. So if we r connecting a cap b/w say Dgnd and chassis gnd. In
this scenario, the noise in the Dgnd will get bypassed to the immediate
chassis gnd ( provides least impedance path for the noise generated from
Dgnd), instead of allowing this noise to the other part of the digital or
analog modules present in this PCB. Pls correct me if I'm wrong .
Thanks& Regards
Sudeep
On Dec 4, 2016 02:35, "Boris Bakshan" <bbakshan@xxxxxxxxx> wrote:
Hi all,
This is a very good discussion we have here. However, I stil haven't
figured out if connecting caps between chassis ground and DGND is a good
practice. Bert Simonovich pointed out hat some designers do that but what
would be the main reason for doing so and what is the merit vs. having one
solid DGND ?
On Nov 29, 2016 00:32, "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx> wrote:
Your definition of ground is correct. Where one hooks one side of ameter
or scope when measuring a voltage.a
I am writing an article on the concept of ground that should clear all of
this up. It will appear on line sometime before DesignCon.
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On
Behalf Of Loyer, Jeff (Redacted sender "jwloyer" for DMARC)
Sent: Monday, November 28, 2016 1:23 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Caps between "isolated grounds"?
Here's what I've concluded, for now...
Regarding the original question, I think the caps were placed between
grounds to allow isolating a ground if a problem arose. I doubt this is
good design practice. Another possibility is that in theory, you might"ground"
want
to have a cap between chassis ground and the ground shield of a very long
wire between two systems, but I haven't found a spec. (for long cables)
that
accommodates that kind of shielding so think it's a moot point. I think
having capacitors between grounds is an indication that someone doesn't
know
what they're doing.
Sorry to disagree, but I'll always have a "GND" symbol in my schematics,
even if I'm designing a satellite. For most signals, it's what I'm going
to
hook my ground lead to when measuring voltages. I know the name's not
perfect, but it beats whatever's in 2nd place. For earth system safety
issues, I might have "earth" to handle lightning strikes, and might also
have a "GND" (uninsulated, or green wire) to ensure a return path for
chafed
cables that short to metal (to force a circuit breaker to pop).
Thanks for the thoughts,
Jeff Loyer
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On
Behalf Of Smith, Larry
Sent: Thursday, November 17, 2016 11:59 AM
To: martin.rowe@xxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Caps between "isolated grounds"?
I have to go with Bruce on this one. We should eliminate the word
from our vocabulary unless we are talking about a place for potatoes anddigital
carrots.
Instead, we should discuss the "return path" when we talk about that node
in
all of our products with zero volts on it. Then when the "money savers"
and
"isolationists" come to signal and power integrity engineers and start
talking about splits we can ask: "What? You are going to cut my return
path? How is the return current going to get between my analog and
sections? Where is my PDN return current going to go? How muchinductance
is that going to add to my loop?"digital
Analog designers often make the argument that sensitive analog circuits
should be isolated from digital by cutting the return path between them.
But the analog sections almost always have to communicate with the
sections with signal lines. If we cut the on-die return path, possiblythe
package return path and heaven forbid the PCB return path, what are thosethis
analog-to-digital signal paths going to do? The A and D reference nodes
are
potentially 100's of mV apart and the noise varies with frequency. Is
what we want? If we are really going to cut the return path between ourcomprehend
analog and digital sections, then we should only use differential signals
for all A-to-D communication.
If we call it what it is - cutting the return path - we begin to
how much damage we are doing to ourselves with return-path splits."earth."
BTW, if we are talking about safety issues, we should use the word
and
Regards,
Larry
PS - These comments apply to signals higher than a few MHz. Sensitive
audio
signals where board traces are not long enough to be transmission lines
long audio cables are a different matter. In these cases, a startopology
makes sense.AC
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On
Behalf Of Martin Rowe
Sent: Tuesday, November 15, 2016 12:39 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Caps between "isolated grounds"?
The term "ground" is probably the most misunderstood and misused term in
electrical engineering. I blame the universities. They start their
electrical engineering instruction with DC circuits and then progress to
circuits with resistors, inductors, and capacitors. But the ideas ofdesigned
parasitic and nonschematic effects are seldom discussed in classes.
Usually,
lab assignments are relatively low-frequency projects -- probably
to ensure parasitic effects aren't encountered.ground
--Bruce Archambeault
http://www.planetanalog.com/author.asp?section_id=3204
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On
Behalf Of Lee Ritchey
Sent: Tuesday, November 15, 2016 3:24 PM
To: dbrooks9@xxxxxxxxxxxxxxx; dmarc-noreply@xxxxxxxxxxxxx;
si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Caps between "isolated grounds"?
Sometimes I am prompted to cite Bruce Archambeault from IBM on what
is.hope
Here is a quote from him.
"Ground is the place where one plants seeds in the spring time in the
that come summer one will get a bumper crop of tomatoes."the
The reason for Agnd and Dgnd on an IC is to isolate the analog side of
IC from the digital side for the purpose of isolating the two circuitsprovide
inside the package, not outside it. They should share the same ground
plane on the PCB. Having two different grounds on the PCB does not
any performance advantage to the IC.not
Outside the IC, efforts should be made to insure the analog source does
share paths with digital signals. We usually do this by using shieldedtrace
cables leading to the analog source.
Hope this clears up some of this confusion.
Lee Ritchey
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On
Behalf Of Doug Brooks
Sent: Tuesday, November 15, 2016 8:33 AM
To: dmarc-noreply@xxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Caps between "isolated grounds"?
Occasionally some people use a capacitor between different grounds in a
misguided attempt to provide to provide a signal return path under a
that crosses over the plane split for signal integrity purposes."grounds"
This, of course, is usually a bad idea. The purpose for separate
in the first place was to isolate noise. All the capacitor does is ensureI've
the noise has a path between the two grounds.
(Note to Jeff: Please send me a current email address off-line.)
Doug
Loyer, Jeff wrote:
I was looking at some designs and found different ground symbolsby capacitors. Can anyone explain why this might be done? Everything
connected
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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