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

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_Wim_:

--- Quote from: zrq on December 29, 2022, 09:12:28 pm ---Today I quickly tried removal of C312 and C318 and do the test exactly as Wim described. The meter appears to settle a bit faster (Orange curve) then before after switching back from Ohms range, around 2x faster, but still needs 13 min to reach 10 fA.

--- End quote ---

Interesting! I will try to make a comparison run related to the popcorn noise when I can spent some time in the lab...

_Wim_:
[ Specified attachment is not available ]
--- Quote from: zrq on December 29, 2022, 09:41:44 am ---One should reproduce on their meters by monitoring the 2V analog out with a deep memory oscilloscope for 600 s and using a timebase suitable for 10 us pulses, maybe for example on 2 nA or  200 mV or other ranges with x10 ranging amplifier gain. Those spikes can have 0.05V to 0.12V amplitude and are actually small bipolar oscillations.

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Hi, I first made a 10min log of the 2V output (with range set at 2nA), but did not see any spike. After that, I configured my scope to trigger on a spike, and let it run over night (the buffer/segmented memory of the Picoscope was configured to store at least 500 triggers).

Upon leaving the lab I saw the first trigger when I turned of the lights. This morning I reviewed the 8 triggers, and I only saw triggers until we left home, and again saw a trigger when we arrived back in the middle of the night.

So, these "popcorn" spikes are spikes that enter via the AC into the 617 and make it through to the 2V output (at least in my case).

zrq:
It seems I have a bigger trouble to solve with my meter before pursuing the noise. After transporting the meter between the day job lab and home lab (public transportation so exposed a little bit to the weather), it got unhappy and the bias cannot settle to 0 fA again even after a overnight warmup. While the bias level is always around 5 fA after stabilization, huge spikes of hundreds of fA in the reading can be appear.
Tried disconnecting relays but later discovered it's probably the triax connector went bad. Unplugging the input immediately reduced the bias back to <2 fA level before a full warm-up. Out of curiosity, tried again Wim's test of charging for 60s in Ohms range and switching back to Amps with the input unplugged. Wow, the connector is also contributing a lot to the "dielectric absorption"! In this configuration the meter settles back to <10fA in 400s.
Well, I have sonicated the connector assembly today and let's see if it gets better after soaked in IPA for a day. Still want to rescue it before paying 107 CHF for the 7078-TRX-TBC.

zrq:
Finally made up mind and modernized the connector to 3-lug triax Pomona 5219, cannot afford a 7078-TRX-TBC. It didn't go through so well, as the hole on the panel is smaller than necessary for 5219. One need to brutally enlarge the hole to insert the Pomona. After I'm reconnecting the wires, I realized there is a possibility that it's not the connector is leaking, it can be the wires connecting the center pin was touching the metal tube for the guard. The coating on the black wire may simply not have good enough insulation. Anyway, already spent the money, so I moved forward. I salvaged some PTFE tubing from a dead gas chromatography vaporizer in a local trash dump, insulated the connection at the panel connector with that and made a simple few-pF PTFE coaxial capacitor replacing C312. While the meter is still nosier than it was before, the crazy bias current is gone and the dielectric absorption is much less obvious. (<7.5 min down to <10fA, doing Wim's test).
Also recapped the electrometer board, although it turned out every cap removed look fine and have rated capacitance.

Now I have a spare Pomona 5090 2-lug triax to BNC adapter and the removed 2-lug triax for sell. Well, probably not many people want them, but let me know if they can help someone.

analogNewbie:
remove one lug, k617 can fit into modern triax connector.

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