Re: [SI-LIST] : Via Capacitances ...

Gregg Fokken (fokken.gregg@mayo.edu)
Mon, 12 Jul 1999 17:06:39 -0500

Mike (Jenkins),

Doesn't Zo*C/2 really only apply if the capacitor is truly a lumped
element?
I still thought of a via as a distributed LC circuit which often "appeared"
more capacitive than inductive, however, if I happened to build a via with
an impedance which matched my line, then I would get no reflection and
according to your formula, no capacitance, when in fact the via still has
capacitance associated with it (only the "ratio" of L to C matches that of
the line). Comments? Thanks in advance for your feedback.

On Jul 12, 2:01pm, Mike Jenkins wrote:
> Subject: Re: [SI-LIST] : Via Capacitances ...
> mcheong@molex.com wrote:
> >
> > I agree with Ron's email on via being nothing but a capacitive stub.
> >
> > One can verify that by measuring a typical via with a TDR.
> > Run a length of trace with an open via in the middle and launch a TDR
signal
> > (~35pS) from one end. Make sure the line is terminated in its
characteristic
> > impedance. You will discover a negative reflection (capacitive
discontinuity).
> >
> > The equivalent capacitance can be derived by utilizing the reflected
voltage,
> > risetime, incident voltage and Zo.
> >
> > Michael Cheong
>
> Michael,
>
> Using risetime to calculate capacitance from a TDR trace is
> problematic since it continually degrades. For a capacitance,
> C, along a transmission line of impedance Zo, the AREA of the
> reflection is the time constant Zo*C/2, exactly. (This can be
> shown from the LaPlace final value theorem.)
>
> Regards,
> Mike
>
> _____________. . . . . . . . ._____________
> / \ _/
> / \ Zo*C/2 __/
> / \________/
> /
> ____/
> --
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> Mike Jenkins Phone: 408.433.7901 _____
> LSI Logic Corp, ms/G715 Fax: 408.433.7461 LSI|LOGIC| (R)
> 1525 McCarthy Blvd. mailto:Jenkins@LSIL.com | |
> Milpitas, CA 95035 http://www.lsilogic.com |_____|
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> **** To unsubscribe from si-list: send e-mail to
majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE
si-list, for more help, put HELP. si-list archives are accessible at
http://www.qsl.net/wb6tpu/si-list ****
>
>-- End of excerpt from Mike Jenkins

-- 
-------------------------------------------------------------------
Gregg Fokken                Internet(email):  fokken.gregg@mayo.edu
Mayo Foundation                       Phone:  (507) 284-5692
200 1st Street S.W.                     FAX:  (507) 284-9171
Rochester, MN  55905  

WWW: http://www.mayo.edu/sppdg/sppdg_home_page.html -------------------------------------------------------------------

**** To unsubscribe from si-list: send e-mail to majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****