[SI-LIST] Return Path
- From: Doug Brooks <doug@xxxxxxxxxx>
- To: darshanmehta2k@xxxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Fri, 05 Aug 2005 10:06:46 -0700
Some of the answers you received may have made this sound more complex than
the issue really is.
First: Current is the flow of electrons. Electrons MUST flow in a closed
loop. The way they find their way "back" is called the "return path." If
you have a simple battery with two wires attached to it, one carries the
outgoing current and the other carries the return current.
Second: Sometimes the return is constrained to a specific wire. Then there
is no question where it is. Sometimes the return is on a plane. Then there
CAN be a question where it is, since it can spread around the plane in a
variety of ways.
Third: WHERE it is on a plane is determined (at least in part) by
electromagnetic coupling between the signal that generates the return and
the return itself. The return will follow the path of least IMPEDANCE. For
high frequency components the lowest impedance path is directly underneath
the trace carrying the signal. For low frequency components, the return
will follow a path of least RESISTANCE, which can be a DIFFERENT path than
for the higher frequency components. Therefore, different parts of the
return path (different frequency harmonics associated with the return
current) may flow back to the source on different paths.
Fourth: Since the lowest impedance path (for the higher frequency
harmonics) is on the plane directly under the trace, this part of the
return current will "want" to be on the "closest" plane, whether it is a
power plane or a ground plane. For HIGH frequency harmonics there is no
difference in these planes. (If that is hard to understand, think of all
the bypass caps there are connecting the two planes!)
Finally: An issue for signal integrity "can" be (depending on
circumstances) how the return current actually gets to the plane it wants
to be on. Sometimes (not always) the way it gets there can cause SI
problems (especially EMI or crosstalk.) It is not unreasonable to state
that MOST of the signal integrity issues on boards today are caused by a
failure to control return currents. Knowing where your return currents are,
and making proper provision for them, is one of the primary ways of
controlling signal integrity.
I cover a lot of this in my book "Signal Integrity Issues and Printed
Circuit Board Design," Prentice Hall.
Doug Brooks
At 10:31 PM 8/4/2005, you wrote:
>Hello Experts,
>
>
>
>I have a question on return current. I was going through the book "Signal
>Integrity Simplified" and came to know that return current path can be a
>Power Plane or Ground Plane. I am still confused about return path. Let me
>describe what I understood.
>
>
>
>The Signal Path is the active path and the Electric field from Signal will
>terminate in the return path. The Magnetic field will form a circular loop
>around the signal and it will be coupled with return path so that equal
>and opposite current will flow in return path. Normally if we have Power
>Plane or Ground place, the return path will be easy to find out. Let's
>assume, if we have a 2 layer board with no copper shape drawn on it, how
>to find the return path for the signal? Please help me understanding this.
>
>
>
>Thanks,
>
>Darshan Mehta
>
>
>
>
>__________________________________________________
>Do You Yahoo!?
>Tired of spam? Yahoo! Mail has the best spam protection around
>http://mail.yahoo.com
>
>------------------------------------------------------------------
>To unsubscribe from si-list:
>si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
>or to administer your membership from a web page, go to:
>//www.freelists.org/webpage/si-list
>
>For help:
>si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>List FAQ wiki page is located at:
> http://si-list.org/wiki/wiki.pl?Si-List_FAQ
>
>List technical documents are available at:
> http://www.si-list.org
>
>List archives are viewable at:
> //www.freelists.org/archives/si-list
>or at our remote archives:
> http://groups.yahoo.com/group/si-list/messages
>Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
>
____________________________________________________________________________-
Check out UltraCAD's new presentation videos and new skin effect calculator
at http://www.ultracad.com
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List FAQ wiki page is located at:
http://si-list.org/wiki/wiki.pl?Si-List_FAQ
List technical documents are available at:
http://www.si-list.org
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
