Robison Michael R CNIN wrote:

> Hello,
> I've drawn a schematic and now I'm attempting to interactively
> route the traces to match some old artwork, but I've run into
> some guard traces that I haven't got in the schematic.  At
> least I think they are guard traces.  They have a via to GND
> at one end and follow beside another trace for a while, and
> then simply stop close by the termination of the actual signal
> trace.

These are Faraday shields, a bit different from a true guard trace.
Generally, true guard traces are closed paths that surround a trace
or several traces, and are charged to a level such that minimal
current can flow through leakage on the board surface or inside the
board to affect the charge of the protected trace.  This kind of stuff
is generally used in ULTRA-high gain amplifiers, and ultra-low
current circuits like electrometers.

Faraday shields ground out the electrostatic field generated by
traces with fast voltage changes, reducing one of the modes of
They don't offer much help for the magnetic field, which causes
crosstalk when large currents are switched on traces.  Having two
bus buffers driving different ends of a trace to opposite polarities
for a few nS is a classic cause of the this.

> To be honest, I don't really care about their electrical func-
> tion.  However, we are attempting to as closely match the orig-
> inal artwork as possible, so the are definitely going on the
> board.
> My question is how do I add these to the board without causing
> a bunch of design rule errors in the end?  Right now I'm simply
> laying the track and punching the via at the end and assigning
> the via to GND.  I know the DRC is gonna hate this, and I depend
> on the DRC to verify my design and I'd like to not have to sift
> thru the DRC to decide what is a valid error and what isn't.  I
> want a clean DRC at the end of my board design.

If you place a track from point to point, it will be listed as
"no net".  Double click on it and through the advanced tab, change
the track to be part of net "GND".  Now, place a via at the end,
and it will automatically connect to the ground plane, assuming this is
a multilayer board.  You can extend the track to follow the geometry
needed.  I don't think this will cause any DRC squawks.  If it does,
they will be of a unique class, and so you can probably know they are
harmless.  If you do get a DRC violation, you can probably set up
special rules to permit this for the GND net.


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