RE: [SI-LIST] : Trace Capacitance

Tim Morley (tmorley@bluews.com)
Mon, 21 Jun 1999 17:50:07 +0100

I suspect this is a case where the "directly proportional" rule is an
approximation that is only accurate if certain conditions are met. In this
case I would expect the condition to be that the separation between the
conductors must much smaller than the linear dimensions of the conductors.
When your trace is 5 mils wide this dimension is not "much smaller" than the
separation from the ground plane.

Tim Morley

-----Original Message-----
From: Farrokh Mottahedin [mailto:Farrokh.Mottahedin@quantum.com]
Sent: 21 June 1999 16:59
To: 'SI List'
Subject: [SI-LIST] : Trace Capacitance

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

**** 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 ****

**** 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 ****