Scott and forum: I'd like to add to the good info that Paul Gould has already given. (This is probably overkill, but you can stop reading anytime you want.) :-) "Characteristic impedance" deals with signals propagating along some uniform path. "Impedance" is a general term defined as opposition to the flow of current and is mathematically the voltage divided by the current at some point in a circuit. It usually applies to components where signal wavelengths are long compared to the dimensions of the circuit, like at lower frequencies. In contrast, "characteristic impedance" is a high-frequency or transient thing and applies to a wavefront as it propagates, like a high-speed (quickly changing)signal along a (relatively long) circuit board trace. It is the ratio of instantaneous voltage to current at the wavefront. It is usually measured with a single-ended (one conductor is ground) TDR (time domain reflectometer). "Differential" has to do with where signal currents are returned through a structure. In striplines and similar geometries, the signal current returns in the ground plane conductor. A signal is usually a voltage referenced to ground. In "differential" situations, the current is returning on another conductor, but not ground. "Differential" refers more to how signals in a multiconductor structure are measured - that is from one conductor to another, where there is a (different but related) signal on each. Usually physical symmetry is involved which allows for "balanced" signals -- equal but opposite with respect to ground (electrical symmetry). Differential TDR's are a convenient way to measure differential impedance. There is essentially one single-ended channel for each of the two conductors being measured. The net signal is the difference between the two, usually done mathematically within the instrument. Two isolated individual 50-ohm single-ended traces when measured differentially would be 100 ohms. I've probably mentioned more than I should have. So if I confused you (or anyone else for that matter), contact me off-line and I'll be happy to try to unconfuse you. All my explanations come with a money-back guarantee. :-) Randall E Rhoads Development Engineer Electromagnetic Technology AMP Incorporated Harrisburg PA USA 717-986-5201 / Telephone & Voice Mail 717-986-5643 / Facsimile [log in to unmask] > -----Original Message----- > From: [log in to unmask] [SMTP:[log in to unmask]] > Sent: Thursday, July 09, 1998 5:52 PM > To: [log in to unmask] > Subject: [TN] Impedance > > Could I please get some feedback as to the definitions and difference > between > characteristic versus differential impedance? I have customers that ask > for > both but can't explain what they are actually looking for. Any help would > be > appreciated. > Thanks, > Scott > > ################################################################ > TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c > ################################################################ > To subscribe/unsubscribe, send a message to [log in to unmask] with > following text in the body: > To subscribe: SUBSCRIBE TechNet <your full name> > To unsubscribe: SIGNOFF TechNet > ################################################################ > Please visit IPC's web site (http://www.ipc.org) "On-Line Services" > section for additional information. > For technical support contact Hugo Scaramuzza at [log in to unmask] or > 847-509-9700 ext.312 > ################################################################ ################################################################ TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c ################################################################ To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body: To subscribe: SUBSCRIBE TechNet <your full name> To unsubscribe: SIGNOFF TechNet ################################################################ Please visit IPC's web site (http://www.ipc.org) "On-Line Services" section for additional information. For technical support contact Hugo Scaramuzza at [log in to unmask] or 847-509-9700 ext.312 ################################################################