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Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617

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Several months ago I purchased a Keithley 617 electrometer (think I'm becoming a Keithley fanboy/collector).  It seemed like a fully functional unit with the exception of readings at the lowest 2 pico-amps setting.  The service procedure gives instructions for setting zero and offset on the electrometer board at the lowest pico-amps setting with no input.  The zero could be set with good stability but the there was too much variability in the 2 pico-amp range several hundred counts.  I did some research/probing but didn't find anything conclusive.  Prior posting:

I held off cleaning the electrometer PCB for fear of adding or spreading contamination - thus adding new or exacerbating existing problem(s).  I finally decided to go for it.  I cleaned the critical half of the electrometer PCB with the best isopopal I could find.  I carefully scrubbed it and flushed it with IPA followed by drying with a heat gun.  I reinstalled the board and let it run for 3 days - It was better but still not there. 

A little frustrated I let it set in the closet for a few months while pursuing other projects.  I recently revisited the zero/offset procedure and surprisingly the unit was much quieter on the 2 pico-amps range.  After ~3-days of power-on, the unit was correctly zero/offset adjusted and is very stable.  This instrument is very sensitive - just moving across the floor makes it jump a couple hundred counts for a few seconds an then settle.

The FIX = TIME.  My best guess is the unit may have absorbed moisture at its previous owners location or while shipping.  It was not shipped in a sealed bag with descendant.  So altitude changes (e.g. air shipping) may have created condensation. 

While searching the web, I found some interesting information on design and cleaning/contamination for this type of instrumentation.
Reference Links:

Cleaning PCB That Are Sensitive To Low Leakage Currents (Great before/after cleaning demonstration):
TI(National) Makes the LMP7721 (3 Femtoampere Input Bias Current Precision Amplifier) and a development board based on this chip. 
Op Amp EVM Cleaning for Femptoamp Bias Currents:

While searching for something else, I stumbled on this thread and found it worth bumping up.

Yeah, very familiar. In my job for an OEM, testing and calibrating measuring instruments that we manufacture, leakage always is an issue. We're talking about an instrument with a specified input impedance of > 1 TOhm.
Cleanliness of the boards is very important. All sensitive boards are washed in an industrial pcb washer using special detergents. This is to remove the spray on flux used in the wave soldering process. This washer also dries the boards, and surprisingly, not much time between washing and testing is needed at all.

We found the biggest contributing factors to leakage were actually what types of plastics were used in connectors in combination with the weather.
Certain types of plastic (like the green Phoenix connectors) can hold quite an amount of moisture and cause leakage in warm, muggy weather that we usually have here in  the summers. The seemingly trivial act of changing to a different type of connector solved a lot of issues we were having during these months.

So, with regard to your meter, just the weather conditions may have an effect on it that's not negligible.

Comment on the video: it looks like a crude way to clean the board, but it's tried and tested, been there done that. That curve on the monitor after cleaning looks very familiar...
Also note that there are no connectors on that board with plastic insulation, just individual pins with air around it. In these low current conditions, plastics turn out not to be that good an insulator at all since it can hold moisture (depending on the type of plastic, as written above).

Also, one of Dave's earlier vids:
sounds pretty familiar. But "my" instruments aren't that sensitive that "someone farting across the other side of the factory" :-DD has a measurable influence...

For most aplications a good cleansing with isopropanol will do. Do let it dry for a while, most people forget/neglect this and doing do can easely generate nanoamps of creepage currents. On your electrometer clean the connectors on both the cables and the device itself too.

Once you play with a (good) electrometer you will soon find out why people use expensive (triaxial) cables, and propper grounding is a must. I have made measurements where it was necisary to keep at least 2 meters distance, not to completely mess up the measurement. Just breathing, walking by or even turning your head or waving your hand can mess it up.

one of the things people do not realise is that, when the cleaner evaporates, it cools down ( evaporation creates a cooling effect ).
this allows moisture form the surrounding air to condense .... moisture that is pretty conductive.

you want to use a mixture of alcohol and diethyl ether for a final rinse (preferably dried under molecular sieves).

and probobly dry it with dry air or nitrogen

what free electron says holds too, waiting for a low humidity condition or using a dry hood/etc helps, you can also heat up the PCB prior to the final clean and then give it the ether rinse


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