Re: [SI-LIST] : Trace Capacitance

Fred Dieckmann (fdieckma@usa.alcatel.com)
Mon, 21 Jun 1999 12:34:50 -0500

If you subtract 5 pF from each measurement, you get 5pF and 20 pF which
give same ratio as the area differences of the traces. Maybe there
is 5pF of stray capacitance in the test set up. The measurements should
be nulled for stray capacitance. Compare the measured results to the
calculated results also to see if they are in the ball park.
Howard Johnson has a MathCad model for capacitance at www.sigcon.com

Good luck

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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