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
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