[SI-LIST] Re: time-constant=2.3*Z0*CL

Pras

If you let a capacitor charge up through a resistor the voltage on the
capacitor will increase in a Vin(1 - e^-t/RC) rate.  Any good book on EE101
electrical theory will cover this very well.  Risetime in many systems is
defined at the time it takes for a signal to go from the 10% value to the
90% value of the 0 to 100% amplitude of the signal.  So, if you start a
stopwatch when the signal passes the 10% point and stop it at the 90% point
it will take about 2.2 time constants or 2.2*RC to make that transition.
The 10% point will occur at about 0.1RC and the 90% point will occur at the
2.3RC point.

Tom Dagostino
Teraspeed(R) Labs
13610 SW Harness Lane
Beaverton, OR 97008
503-430-1065
503-430-1285 FAX
tom@xxxxxxxxxxxxx
www.teraspeed.com

Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
401-284-1827 


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Pras venki
Sent: Friday, December 14, 2007 10:42 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] time-constant=2.3*Z0*CL

Hello there,
In the equation for "time constant of the rise-time", when a trace is
terminated by a load capacitance-

time-constant=2.3*Z0*CL

what does the factor 2.3 stand for and when should it be used? How is it
derived?

Thanx in advance.

-Pras


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