I would like to build an amplifier can serve as an electrometer (measuring voltage with very low input current/high input impedance) at high bandwidth, ideally up to 10MHz. Think of it as a hybrid between an electrometer and LNA :) Desired specs: input current as low as possible (femtoamps), noise as low as possible, bandwidth >10MHz. This is the front end of a custom instrument in which I am trying to measure microvolt signals from a source with impedance >100MOhm at 5-10MHz. The source is physically small, fits in a 1mm cube.
There seem to be at least two big problems with this:
The standard electrometer-grade op-amps have very low bandwidth, for example OPA129 has offset/bias current of 30fA but unity gain bandwidth of 1MHz. (Another device, the LMP7721 looks nice with 3fA offset and 17MHz bandwidth)
I assume the source would have to be very close (essentially located between the input pins of the op-amp), but it seems even the tiny parasitic capacitance would be enough to short-circuit a lot of the signal at higher frequencies.
Any suggestions for how to approach this problem?
Update: Thank you for the helpful discussion. I am not completely sure of the source properties, I made an order of magnitude guess from a rough physical model. What I do know is that most simple amplifier configurations hooked up to that source show no signal, so the signal has got to be very weak. It appears that accurately modelling the source is key for making any progress.
I am aware of thermal noise, and indeed the signal I am looking for may be quite weak compared to thermal noise. It may be possible to see it by integrating over time (after digitizing) despite that.