Re: Shielding

Fred (fred@contec.apsimtech.COM)
Wed, 2 Jul 1997 09:42:15 +0800

I concur with most of the discussion on guard traces. On digital circuits where
crosstalk is a concern spacing the traces is an effective and easier way. However
in analog circuits where signal to noise ratios can be quite high guard traces would
give one better noise immunity on very sensitive lines. This assumes that the quard
traces are done properly. At high frequencies this means stiching in ground or power
vias. Driven guard traces on input of op-amps do work especially well when the
driving op-amp is fast enough compared to the design. For EMI guard traces can
reduce the loop inductance when planes are split. If there are no split planes then
use of guard traces to reduce emi is not only useless but may cause problems as
long guard traces are very good antennas. In general guard traces should only be used
by experienced SI engineers who can evaluate if and when their use does any good.
Most likely this happen to be in analog circuitry.

Best Regards,

Fred Balistreri
Applied Simulation Technology