[SI-LIST] Re: Should the signals always return back through GND
- From: "Lynne D. Green" <lgreen22@xxxxxxxxxxxxxx>
- To: <hermann.ruckerbauer@xxxxxxxxxxxx>, <a.ingraham@xxxxxxxx>, <nikitanivan@xxxxxxxxxxx>
- Date: Tue, 23 Aug 2005 09:25:43 -0700
Hello, Hermann,
Shielding can cause new problems. A shield trace can pick up crosstalk, and
then couple that crosstalk onto a third trace. I know of at least one
design that failed in this manner. The secondary coupling was in an
unexpected area of the board, making it hard to debug.
There are two common shielding approaches. First, one could remove the
shield trace and leave the larger trace spacing in place (decreasing the
capacitance between traces). Second, one could add enough vias (to the
desired DC voltage) to make sure the shield does not conduct crosstalk noise
very far.
Good SI tools can be used to check the crosstalk conduction and secondary
coupling.
Best regards,
Lynne
PS: more history on GND: Breadboard circuits had much higher impedances in
the circuit that in the connecting wires, so students (and junior engineers)
treated all points along the wire as GND. Given this earlier training,
learning distributed transmission line theory was very difficult for most of
them.
"IBIS training when you need it, where you need it."
Dr. Lynne Green
Green Streak Programs
http://www.greenstreakprograms.com
425-788-0412
lgreen22@xxxxxxxxxxxxxx
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of hermann.ruckerbauer@xxxxxxxxxxxx
Sent: Tuesday, August 23, 2005 2:14 AM
To: a.ingraham@xxxxxxxx; nikitanivan@xxxxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Should the signals always return back through GND
=20
Hi Andy,
Good answer!
That points me to one question that I have had quite a while ago ...
Just from a high level point of view I would try to get the same current
returen for DC and AC. So If I do have a high level terminated signal (e. g.
to VDD) I would try to do the referencing/shielding also with VDD to avoid
any referencing/return crossing even if it is just between DC
and AC (btw. What is DC what is AC in this discussion ;-) )!
Of course I might get some disadvantages by doing so. Usually the most
stable signal is called GND (whatever it really is ...). So any referencing
to VDD might disturb my signal by talking to it due to any noise on the
reference!
What is your/the groups opinion on this ?
Thanks
Hermann
=20
>
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Andrew Ingraham
Sent: Monday, August 22, 2005 6:24 PM
To: nikitanivan@xxxxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Should the signals always return back through GND
> Its always said that GND acts as a return path for a signal.
Really? I wasn't aware of this. If everyone says that where you come from,
then maybe you should educate your teachers and/or co-workers that they are
wrong.
What we call GND is often used as a reference point for VOLTAGE, but this
doesn't mean it is where CURRENT flows.
> Do all the signals know that they
> have to return back through GND and not through anyother track in
> the=20 signal layer having lower impedance? What are the factors taken
> into=20 consideration to make sure that signals return back through GND.
At low frequencies and DC, signal currents return to wherever they came
from, to complete the loop.
If you take a signal source and connect one end of the source to GND and the
other end of it to a wire that goes off somewhere to some load, then the DC
return path will have to get back to GND to complete that loop.
If you take that same source and connect the first end of it to VDD rather
than to GND, then the DC return path will have to end up at VDD and not GND.
It might go by way of GND in order to get back to VDD, but that depends on
the topology, the load, etc. The exact path (GND plane vs. VDD plane, etc.)
that it takes, depends on the relative DC resistances and low frequency
impedances of those paths.
Thus, when the pull-up transistor in a pull-up/pull-down (or
"totem-pole") pair is on, the return path (at DC and low frequencies) will
have to get back to the VDD supply net that connects to that output driver.
GND might not be involved at all.
At high frequencies, signal switching current return paths for a wire or a
trace, are by way of any and every conductor that is nearby, to which field
lines can be drawn. Most of the high frequency current chooses the path(s)
with the lowest impedance. The signal propagates via an electro-magnetic
field in the dielectric, which just happens to cause currents to flow in all
conductors that touch the E-M field.
If a trace runs over a solid plane that connects to, say, some VTT voltage,
then return current will be in that VTT plane. If a trace runs between two
planes, the high frequency return current will be shared between both of
them, regardless of what voltage each one is connected to. If a trace runs
over one plane for several inches, then a different plane for several
inches, and so on, then the return current will have to (or try to) find its
way on each of these planes, in series.
The DC path may take a totally different route. That is, the initial
switching wavefront has a return path associated with it, determined by the
E-M field around and between the conductors; but after a few nanoseconds the
return current may take a different route, eventually determined by DC
resistances and not the E-M field anymore.
Regards,
Andy
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