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

[Karel Kulhavy <clock@xxxxxxxxxxxxx>]

On Fri, Mar 24, 2006 at 06:41:13AM -0800, Steve Meier wrote:
> 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

Yes because between the tip and the ground of the probe it's an
unshielded wire and it picks up garbage from the air.

The same happens when I connect the tip and the ground of the probe
without connecting it to anything else.

> resistence and an infinite supply of available electrons this noise
> wouldn't be there. Since ground is the return path of most circuits

Would, because it's there even without the ground under test.

CL<
> (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<
> >