Yup - Keithley 640 - that must be the one. This stirred my memory somewhat, and I just located info on the model 642 also, which was apparently newer. The 642 went to (or back to) ultra-low bias MOSFETs, while keeping sapphire insulation and a separate input head. The MOSFETs need all kinds of offset, temperature, and bias current compensation. The 642 also uses a few digits of DVM that obscure the real capabilities, as I mentioned previously. The specs apparently show the most sensitive range as 1 pA FS, so all stuff below E-12 A depends on those digits to resolve. The manual recommends that extremely small currents below 1 fA (the third digit down) be measured in charge mode. I think this is to compensate for bias current and to average out some of the 1/f noise.

The 640 on the other hand, can apparently reach 1 fA FS (1000x lower) with 100 aA p-p (+/- 5%) noise on analog readout. Given a choice between the two, I think I'd pick the 640, and hook a DVM to the output, and average a whole bunch if necessary. I think the 640 uses a superior front-end technology that maybe could be even further improved in the middle and back end, while the 642 probably is as good as it can ever be already.

Ed

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