This is more of an analog question but I'd thought I'd pick the collective wisdom here. I'm working on recovering AC signals from some very small capacitors, typically 100 to 300fF (Yep, femtoFarads, 1 X 10^-15F). The capacitor is driven by a small amplitude RF sine source, the other terminal is loaded with a high value resistor, typically selected to be about equal to the reactance of the capacitor under test at the RF excitation frequency. Simple circuit, right? Problem is, anything I connect to the junction between the capacitor-under-test and the resistor load has waaaay more stray capacitance then the capacitor of interest, the result being that the stray C in parallel with the load resistor load drops the effective load impedance, and the SNR goes kaput in a hurry. Even op-amps with very high bandwidth can have differential and/or common mode input capacitance of 2 to 4 pF. I've used active parasitic capacitance canceling circuits on MOS microprobes and some active O'scope probes, but has anyone here actually seen one of these active capacitance cancellation circuits? MOS microprobes typically have <40fF of input capacitance, 10^-14A of input leakage and bandwidth to 300MHz; I'd love to get my mitts on a circuit that would perform even close to these specs, build it and play with it some. Regards, Gary M. ------------------------------------------------------------------ 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