> By definition the impedance of a capacitor is 1/(j * omega * C). It is > not constant. The best you can do is to adjust C to place the > capacitance within the range you need, over the bandwidth you desire. Scott, my read is he's not talking about that capacitance and not that impedance. For a DC blocking application, C is large so its impedance over the useful range of frequencies contained in the signal ought to be much less than Zo, not matched to it. Anyway, we all know from transmission line theory that, even though 1/(j * omega * C) is not constant (where the C is capacitance per unit length), the Zo of the line is moderately constant. I think he is talking about the mutual 'stray' C between the capacitors. That, combined with their mounted inductance, results in some effective impedance of the mounted capacitor pair to differential signals. If the board uses a pair of 50 ohm loosely-coupled co-planar traces, then one might want the capacitor pair to have an effective 100 ohm characteristic impedance to differential signals, treating the capacitors as a short differential transmission line. That's got nothing to do with passing your lowest frequencies of interest. Except that capacitor value influences construction and inductance. > does anyone have a > different approach to this problem? Adjusting pad sizes on the board may help as you approach (but not encroach into) the frequencies where the capacitors need to be treated as a transmission line. Andy ------------------------------------------------------------------ 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 technical documents are available at: http://www.si-list.net 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