The best method is to give the customer a SPICE model and let him figure out the output impedance based on his frequency of operation and application. The method I prefer ties an ideal open transmission line to the driver. I like to make the length a few inches, but long enough so it is easy to see the voltage transitions at the end of the line. Then solve for Zout: Vout = 2 x VDD x Z0 / (Z0 + Zout) where Vout is the pedestal at the end of the line and assuming VDD for a CMOS driver (for other driver types, use the voltage that would be used in a Thevenin equivalent of the driver) Using a resistor to ground rather than an open transmission line keeps the DC currents high, which can change the output impedance. In most applications, the DC currents are low, but use a topology that makes sense for the application. Hence, I prefer getting the SPICE model. Dennis ----- Original Message ----- From: "eric steimle" <ericsilist@xxxxxxxxx> To: <si-list@xxxxxxxxxxxxx> Sent: Wednesday, February 23, 2005 6:44 PM Subject: [SI-LIST] Driver Output Impedance am I doing this correctly? > Hi, > Thanks in advance for anyone taking the time to read > this, I know how valuable everyone's time is. I'm > trying to calculate a number for our driver output > impedance that I can give to someone working with our > chip. I'm relatively new at this so I was wondering > if my approach was flawed. > > My configuration is: > > Driver --> BondWire --> Substrate Board Trace --> Ball > > My driver's rise time is 500ps and my goal is to > calculate our total impedance value in order to allow > others to choose the appropriate series terminating > resistor. > > Driver > 1.) I used spice with a 50 Ohm load on the output and > Ohms law to get the real impedance of the driver > Zdriver (14.5Ohms). > > BondWire > 2.) I read through the SI archives and other docs and > then I assumed the inductance of the bondwire was > 1nH/mm on a 2mm bondwire. That means: > > BondWire = 2nH > > Then I took the Frequency of the output to be: > F = .35/Tr > F = .35/500ps > F = 700Mhz > > Then I used the impedance formula for an inductor (can > I do this with the F I calculated or does that not > make sense?). > > Z = 2pi * f * L > Z = 2pi * 700Mhz * 2nH > Z = 8.8Ohms > > Finally > So I figured that the Zdriver + Zbondwire +Zimag(I'm > missing that) would equal the output impedance of the > driver itself, and I know the impedance of the > substrate trace. > > Does that make sense, or am I going about this all > wrong? I still don't know how to calculate the > frequency dependant part of the driver output > impedance but I'm researching like crazy. > > Any feedback or new reference material would be > greatly appreciated. > > Thanks, > Eric > > > > __________________________________ > Do you Yahoo!? > Take Yahoo! Mail with you! Get it on your mobile phone. > http://mobile.yahoo.com/maildemo > ------------------------------------------------------------------ > To unsubscribe from si-list: > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field > > or to administer your membership from a web page, go to: > //www.freelists.org/webpage/si-list > > For help: > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field > > List FAQ wiki page is located at: > http://si-list.org/wiki/wiki.pl?Si-List_FAQ > > List technical documents are available at: > http://www.si-list.org > > List archives are viewable at: > //www.freelists.org/archives/si-list > or at our remote archives: > http://groups.yahoo.com/group/si-list/messages > Old (prior to June 6, 2001) list archives are viewable at: > http://www.qsl.net/wb6tpu > > ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu