the leakage from adjacent traces referred to here (analog) is surface leakage
due to humidity and that is what guard rings are for around FET inputs on
sensitive op-amps.  Follow recommended layout in the Analog Design or Burr-Brown
or Linear Technology or National Semiconductor data books for those kinds of
circuits.

Bob Landman
H&L Instruments

----- Original Message -----
From: Matthias Mansfeld <[log in to unmask]>
To: <[log in to unmask]>
Sent: July 27, 1999 5:39 AM
Subject: Re: [DC] Ground Shield Tracks


> On 27 Jul 99 at 9:07, Hawes, Adam wrote:
>
> >         I feel sure this is probably an issue for a lot of PCB
> > designers......
> >
> >         Recently, I have come across an increasing number of engineers who
> > wish to have certain tracks on multilayer PCB's shielded by ground traces
> > running adjacently. It seems to be a case of jumping on the bandwagon to me
> > because we never used to do this until about 12 months ago.
> >
> >         I was under the impression that ground tracks only served a useful
> > purpose on 1or 2 layer boards and that on a multilayer they were essentially
> > a waste of space as most of the capacitive coupling would be straight to the
> > ground plane.
>
> Depends on what the circuitry does:
>
> If your shielded traces carry RF signals and/or if you try to build
> something with controlled impedance, then the shielded traces act
> like a coaxial cable and you should get indeed the same or
> better results on a well defined multilayer buildup with a power or
> better ground layer.
>
> On the other side, if these shielded traces have to carry very low
> analog signals to high impedance inputs (e.g. FET OP Amp inputs),
> then these shielding tracks may have the purpose to "catch" leakage
> currents from adjacent traces.
>
> It would be necessary to have a deep look in the schematics and
> perhaps in the application notes for the components used in this
> circuitry.
>
> HTH
> Regards
> Matthias Mansfeld
> -----------------------------------------------
> Matthias Mansfeld Elektronik
> * Printed Circuit Board Design and Assembly
> Am Langhoelzl 11, D-85540 Haar, GERMANY
> Phone: +49-89-4620 0937, Fax: +49-89-4620 0938
> E-Mail: [log in to unmask]
> Internet: http://www.mansfeld-elektronik.de
>