Re: [SI-LIST] : Trace Capacitance

Jim Freeman (freeman@mavericknetworks.com)
Mon, 21 Jun 1999 10:25:15 -0700

Hi Farrokh,
Ther are two components to capacitance, fringing fiels and parallel field. Since the two lines are exactly the same in construction, the fringing fields are a constant offset for the measurements. You can form a plot fo the
two measurements for capacitance vs. conductor width. The w=0 intercept will be the fringing field capacitance offset and when the two measurements are then compared with the fringing field offset subtracted, the parallel plate
capacitance should yield a 4:1 ratio.

Jim Freeman

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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