Joe, I might answer this question rhetorically: At what trace length, frequency, and trace impedance? The college textbook answer is: Zi = Z0 (ZL + Z0 tanh (w l))/(Z0 + ZL tanh (w l)) where Zi= input impedance to a loaded transmission line Z0= trace impedance ZL= load impedance w= propagation constant l= length of the trace Two simplifications: l is very short -> Zi= ZL l is very long -> Zi= Z0 I would try to use the simplifications to compute a short trace scenario and recognize that input impedance will be Z0 for long lines. BTW, I find the Intro to Electromagnetics books from college to be somewhat helpful. The only catch is that many of these books are written with sinusoidal steady state in mind. When working with digital pulse trains, you would have to compute these impedances at every frequency that exists in a psuedo random pulse train. The easiest and best answer probably is to simulate using some spice like simulator and then looking at the current that enters the trace (T-line segment). You will notice that this current may not look like what you might expect from the theory, depending on your termination scheme and the size of your parasitics. This current will give you the best indication of what kind of current drive capability that your drivers will need to have. Hope this helps, James J. -----Original Message----- From: Joe Young [mailto:joeyoung10@xxxxxxxxx] Sent: Monday, April 15, 2002 4:53 PM To: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Impedance calcuation Hi Experts, I have a very basic (silly?) question in my mind. I need to calculate the total load or impedance faced by the Driver.I know the input capacitance of the receivers. And also i know/decide the trace length information. Device1 --- 8.8 nH , 1.0PF Device2 --- 6 nH, 2 pF Device3 --- 8 nH, 7 pF Device4 -- 3.3nH 0.7 pF Device5 -- 3.3nH 0.7 pF and assume device 2 is 1 inch from device 1 and device 3 is 2 inch from Device 1(1 inch from Device 2) and Device 4 and 5 are 4 inch from device 1 (2 inch from device 3) Device 1 drives Device2 to Device 5. I have to come up with a best topology. Device 2 operates at 83 Mhz and Device3 to 5 operates at 33 Mhz. Now i want to know the impedance seen by the Device 1 when it is driving the loads. Ofcourse, i can refer the books. But i would like to know is there any easier formula for come to quickest conclusion to decide th topology? Any thumb rule?.Reference to any formula?. Am i missing any parameter that has to be included? This question may be a very basic and can be found in books but i would like to hear from the Experts to know how they would decide. Expecting the inputs from the experts to add up my experience. If anybody could give me a detailed paperwork, really it would be helpful for me to do further experiments. rgds, Joe See Dave Matthews Band live or win a signed guitar http://r.lycos.com/r/bmgfly_mail_dmb/http://win.ipromotions.com/lycos_020201 /splash.asp ------------------------------------------------------------------ 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 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 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