Re: gEDA-user: basic anti-EMI design q

[Steve Meier <smeier@xxxxxxxxxxxxxxxxxxx>]

Ideally true but in reality not true. Put you oscilloscope probe tip to
the ground plane at a point further then the probes ground. More often
then not you will see some sort of noise. If the ground had zero
resistence and an infinite supply of available electrons this noise
wouldn't be there. Since ground is the return path of most circuits
(obviously not differential circuits) and two or more circuits can have
return paths that cross each other on the ground there will be cross
talk on the ground plane. In the case of digital circuits which often
have high frequency clocks that clock itself can propigate through the
ground and power and into the analog circuits where it can then be
amplified causing distortion.

So it is often recomended using two grounds which are connected at a
single point. One for analog one for digital. The question is often
where and how to connect the two grounds. My point is that putting in
the pads for a quality power inductor gives one options. 1) use an
inductor 2) short the inductor 3) leave it open allowing one of the
grounds to float. I have found that using a good inductor does attenuate
the digital signals, and signals from things like switching power
supplies before they get on the analog ground.

Your experiences may very.

Steve Meier
>As the ground is thought as a zero point, nothing can propagate through
>it.
>
>Unless the ground has resistance or inductance, but then it's not a ground.
>
>CL<
>