Unfortunately you didn't manage yet to tell your intentions in an understandable way, also in your previous thread, where you had sketched a charge amplifier diagram.
Particularly, I don't understand the meaning of the 5000 S/s rate. A current of 1 fA (10e-15 A) correspondends to 6250 electrons per second. Sampling the measurement with 5000 Hz results in about 1 electron per sample. So you are actually talking about counting single electrons. It's possible e.g. with channel electron multipliers, but not with charge amplifiers, I fear.
Currents in a fA range can be measured with regular amplifiers in much slower time scale, with about multiple seconds measurement time.