[SI-LIST] Re: Package, pad, bond, and bond wire parasitic capacitence USB 2.0 HS

  • From: "Ingraham, Andrew" <a.ingraham@xxxxxxxx>
  • To: <si-list@xxxxxxxxxxxxx>
  • Date: Tue, 17 Feb 2004 11:31:53 -0500

> ... I was hoping
> someone could explain to me what causes the above
> mentioned capacitances. I've been reading and
> googling and so far I found a lot of information
> saying they exist but not a lot explaining why (and
> perhaps even how they change with increasing
> frequency).

Maybe I've over-simplified your question, but ...

Capacitance exists everywhere, between every conductor and every other
conductor in the universe.  Most of it is extremely insignificant and can be
ignored; but it is strongly dependent on separation.  Anything close to
another conductor or ground plane has capacitance to it.  Long, closely
spaced lead frames, bond wires, and pins have more mutual capacitance.

Mathematically, it's done as an integration; you break conductors down into
lots of (infinitesimally small) elements, then add up all the contributions.
That's essentially what field-solver programs do.

There's also inductance in series with wires; and when you take the two
together, what you get is an effective capacitance (when measuring it at one
point) that varies with frequency.  The capacitance you measure is the sum
of all the little capacitances of all the segments that make up the
conductor ... altered by the inductances and/or resistances between those
pieces and your measurement point.

It gets bad near resonances, where the frequency dependent capacitance
approaches infinite.

Regards,
Andy


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