On Jun 21, 9:53am, jonp@pacbell.net wrote:
> Subject: Re: [SI-LIST] : Trace Capacitance
> The answer to this should be "Fringing capacitance".
>
> You have some amount of capacitance associated with the sides (and top) of
the conductor. This extra C is more or less a constant delta no matter how wide
the trace is so it is more visible in narrow traces. To take your example,
> If you assume that the fringe capacitance is 5pf, then the we get 5pf for 5
mils and 5 pf for fringe = 10pf.
> Then at 20 mils we get 5 pf for fringe and (4 * 5pf (4*5mils = 20)) = 25pf
for 20mils.
>
> I think it was a little lucky that the numbers worked out, but you get the
idea. You can see this real well if you look at a 2D extraction with field
lines.
>
> regards,
> jon powell
> Viewlogic Consulting Services
>
> Farrokh Mottahedin wrote:
>
> > I hope someone can explain what is happening with the following capacitance
> > measurement. A friend asks if capacitance is directly proportional to
> > surface area, why doesn't his measurement scale linearly. On a pcb made
> > specifically for trace characteristics measurement, he measures
> > 1) 10 pf for a 10 cm long trace that is 5 mils (.12 mm) wide
> > 2) 25 pf for a 10 cm long trace that is 20 mils (.50 mm) wide
> > 3) Both traces are far apart from each other and from other components.
> > 4) Both traces are over a solid ground plane.
> > 5) Equipment is HP 4285A Precision LCR meter set at 1 MHz
> >
> > Thanks,
> >
> > Farrokh Mottahedin
> > Quantum Corporation
> > (408) 324-7934
> > farrokh.mottahedin@quantum.com
> >
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