Oops, yes. Thats what you said. Differential impedance = 100R
Roland 2009/10/10 Roland Jollivet <roland.jolli...@gmail.com> > > 2009/10/10 Jon Elson <el...@pico-systems.com> > >> Roland Jollivet wrote: >> > A twisted pair, or any long piece of wire coming out of the back of a >> PC, is >> > first and foremost, a transmission line, and needs to be terminated >> > properly. This has nothing to do with the DC bias. The impedance of such >> a >> > line is generally 50R to 75R. >> > >> Actually, twised pairs usually come out to about 110 Ohms. The ratio of >> wire diameter to wire >> spacing changes it somewhat, but for normal signal wires just twisted >> together without additional >> spacer between, the differential impedance is almost always going to be >> between 100 and 120 Ohms. >> > > Not quite. As you say below, with a balanced line one could terminate each > line in a 50R to ground. This means that actually, each line is 50R, but > they 'meet in the middle', and that's why one often uses a single, floating > 100R. So, a balanced line is actually 2 x transmission lines, each of 50R, > that terminate back to back. > > Yes, there are variances and peculiarities, but my gist is that a better > understanding of what is happening is preferable to simply plugging > components until something works or pops. > > > Roland > > > > > > >> > In the case of a proper balanced line, there is usually a single low >> value >> > resistor (50R) across the line as it enters the differntial input of the >> > amplifier, but thats not the case here. >> > >> You will sometimes see a split termination, with two 50 Ohm resistors to >> ground or other >> voltage supply, the differential impedance would then be 100 Ohms. >> > Trying to satisfy both AC and DC requirements with a single resistor, >> may >> > either overload the port line if it is too low, or allow signal >> reflections >> > if it too high. >> > >> > The line should be terminated with a pull-up according to the power >> output >> > of the driver chip, then a scope used to select the correct RC >> combination. >> > The RC connects the line to ground. Use a 100R resistor, and start with >> cap >> > order of 50pF and increase it until you see a nice square wave. >> > >> Yes, scoping the final result to make sure all the transient energy is >> completely absorbed >> is a good idea, if you are so equipped. Because of inductive and >> capacitive effects of all >> components, the "ideal" resistor value may not perform the best, and >> some adjustment to get >> the minimum ringing may be needed. >> >> >> Jon >> >> >> ------------------------------------------------------------------------------ >> Come build with us! The BlackBerry(R) Developer Conference in SF, CA >> is the only developer event you need to attend this year. Jumpstart your >> developing skills, take BlackBerry mobile applications to market and stay >> ahead of the curve. Join us from November 9 - 12, 2009. Register now! >> http://p.sf.net/sfu/devconference >> _______________________________________________ >> Emc-users mailing list >> Emc-users@lists.sourceforge.net >> https://lists.sourceforge.net/lists/listinfo/emc-users >> > > ------------------------------------------------------------------------------ Come build with us! The BlackBerry(R) Developer Conference in SF, CA is the only developer event you need to attend this year. Jumpstart your developing skills, take BlackBerry mobile applications to market and stay ahead of the curve. Join us from November 9 - 12, 2009. Register now! http://p.sf.net/sfu/devconference _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
