[SI-LIST] Active capacitance canceling circuitry

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.

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