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

[rastro]

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: https://www.eevblog.com/forum/testgear/keithley-617-current-input-adjustment/msg450438/#msg450438

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:

http://www.edn.com/design/analog/4368681/Design-femtoampere-circuits-with-low-leakage-part-one
http://www.edn.com/design/analog/4375459/1/Design-femtoampere-circuits-with-low-leakage---Part-2--Component-selection

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:

[jitter]

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" has a measurable influence...

[Smith]

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.

[free_electron]

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.

[sarepairman2]

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

[Alex Nikitin]

I've joined this forum because of the Keithley 617. I've acquired one recently just for fun (I work with electrometers daily and with low current stuff in general for many years) and found several threads here about this device. My one came in a very good working state, with the input current meeting the specs (<4fA after a warm-up). However I've worked with the 617 before and it has one particular side which annoys me - the offset temperature drift. The input dual JFET used meet the leakage current specs but not very good in the offset voltage temperature stability. I thought about a cheap way to improve the stability and (possible) the input current level too, by replacing the dual JFET with a dual opamp - the venerable (and cheap) LMC662. The result was excellent in the DC offset stability, however I couldn't compensate the input current fully. After an investigation I've found that my 617 is an early unit, from about 1984, with a different pcb, a different input JFET and without the input protection transistors (compared to the service manual available on the web)! It also features a different input current compensation circuit, working in one polarity only (negative). For the LMC662 I had to put compensation to minimum (essentially none) and was left with about -2... -3fA cold and -7... -10fA after a warm-up. I will try to update the compensation circuit later today (making it a dual polarity as in later units) and see how stable the input current with the opamps in place of JFETs will be when properly compensated. I may also add some diode protection to the input however the opamp should survive in most overload conditions without it.

Cheers

Alex

[Smith]

I've just cleaned a 617 yesterday, after it failed calibration. 2pA was unusable, it had more than 450fA offset and more than 80fA noise (this should not exceed 15fA). Totally cleaned the input PCB, and replaced 2 bad caps and a bad potentiometer. It now works like new.

I've posted some pictures on TiNs FTP.

[rastro]

Hello Smith;

Thanks - that's good stuff for future reference. Could you tell us which 2 capacitors and potentiometer?  What was was wrong with them (leak, ESR, capacitance)?  What led you to suspect these 3 parts?
 Also I have no idea how to access TINs FTP - wouldn't it be better to post a couple of pictures here?

 -rastro

[yodhe]

FTP details are on the xdevs site, but I don't know if you can access the uploads.

https://doc.xdevs.com/article/keithley-instruments/

[Alex Nikitin]

Here is the input bias current of the Keithley 617 with LMC662A in place of the input JFET pair for over an hour after I've adjusted it. I'll leave the logger to work overnight to see how the bias behaves.

Cheers

Alex

[TiN]

There is no http mirror for uploads, but I usually check FTP often. 

Here are Smith's photos, thanks!

[rastro]

Here is the input bias current of the Keithley 617 with LMC662A in place of the input JFET pair for over an hour after I've adjusted it. I'll leave the logger to work overnight to see how the bias behaves.

Cheers

Alex

Alex,
If I understand correctly, you've replaced the (unobtainable) Q308 low leakage dual JFET with an ultra low bias current op-amp??!!  The spec's on the LMC662 look very impressive but I did not think an op-amp would be an easy fit for the dual JFET.  I'd be interested in hearing more details on your modifications...
-rastro

[Smith]

Could you tell us which 2 capacitors and potentiometer?  What was was wrong with them (leak, ESR, capacitance)?  What led you to suspect these 3 parts?
 
 -rastro

I've replaced R348 which is used for setting the input current. It was 5K 3/4 turn pot instead of 10K used in the manual. I replaced it with a small Bourns 10K multi turn (don't know the type) and by offsetting the pins I could still set it via the original hole in the shielding (sorry, didn't make a picture afterwards). Its much easier to set a 10 or 13 turn pot than a 3/4 turn pot. Turning the old pot did nothing.

When I was at it I checked all old electrolytics because let's face it, its build around 1984. Most where fine, but two of them had quite a high ESR (capacity was fine). It were the two identical blue Ritchey (1000uF 16V if I remember correctly) caps next to the transformer on the (smaller) input PCB. I don't know the number as there was no silk and I could not find a silkscreen layout of the input board.

Unfortunately I had no time reading the EEPROM, it was version 4.2 as I remember correctly.

I am interested about the opamp conversion too. Maybe it's something for my Keithley 602 which I still have to repair. The opamp looks quite promising. Just looked at the temperature specs, and input bias should only be 2pA 2fA at 25C, and 4pA 4fA at 50C. I don't think this electrometer will hit 50C internally (although there are no ventilation holes).

[Alex Nikitin]

Alex,
If I understand correctly, you've replaced the (unobtainable) Q308 low leakage dual JFET with an ultra low bias current op-amp??!!  The spec's on the LMC662 look very impressive but I did not think an op-amp would be an easy fit for the dual JFET.  I'd be interested in hearing more details on your modifications...
-rastro

Fortunately, in the 617 the input FETs are just followers, so I've used opamp as two follower buffers. Works fine. 18K resistors are needed to imitate the FET's output impedance, these should be selected to match within 0.1% .The orientation of the opamp is important - pin 5 is better to use as the input pin as it is far away from the supply pins! Below is a part from the 617's circuit and a sketch of my replacement. as well as the overnight measurement of the input current (the temperature in the lab went down by about 6-7C during the night). The input current stayed almost in spec even in these conditions. I've also added 220nF film cap between the supply pins of the opamp (visible on the photo I've posted earlier), this cap could be 47nF-220nF in value, preferably film.

Cheers

Alex

[Alex Nikitin]

I am interested about the opamp conversion too. Maybe it's something for my Keithley 602 which I still have to repair. The opamp looks quite promising. Just looked at the temperature specs, and input bias should only be 2pA at 25C, and 4pA at 50C. I don't think this electrometer will hit 50C internally (although there are no ventilation holes).

Femtoamps! Real input currents of the LMC662 are in a low fA region (if you keep it clean, obviously).

Cheers

Alex

[Smith]

Femtoamps! Real input currents of the LMC662 are in low fA region (if you keep it clean, obviously).

You are right, I was talking about femto amps. I might give this a try. I have some other older Keithley gear with blown inputs featuring unobtainium components.

I was wondering what other people here are using electrometers for. I use them for very low current measurement on light cells, high ohms resistor and leakage measurements (10M to >10Gohms and for measurements in circuit (high impedance voltage). I must admit I occasionally use their voltage source as a voltage supply or calibration voltage. 

BTW Alex, I wondered how you determined the FET output impedance resistor as 18K. I would imagine this value to be quite a lot lower.

[Alex Nikitin]

BTW Alex, I wondered how you determined the FET output impedance resistor as 18K. I would imagine this value to be quite a lot lower.

These FETs have (roughly) the transconductance of about 2mA/V at 1mA (~0.5K output impedance in a follower). In the 617 circuit these FETS are run at a rather low current (~25uA). The transconductance in a JFET is (again, roughly) proportional to the drain current, so for 40 times lower current the output impedance would be about 40 times higher or ~20K. These resistors are needed as the second stage on the LT1012 is an integrator relying upon the previous stage output impedance.

Cheers

Alex

[Smith]

Thanks for the explanation 

[rastro]

I see the LMC662A is available in a through-hole MDIP-8 package.  This might be a better choice over the SMD version.  I don't think the board is too cramped to accommodate the larger package.  It would also be easier to solder and clean up.  Also it seems like there would be more distance or physical isolation between the pins.

-rastro

[Alex Nikitin]

I see the LMC662A is available in a through-hole MDIP-8 package.  This might be a better choice over the SMD version.  I don't think the board is too cramped to accommodate the larger package.  It would also be easier to solder and clean up.  Also it seems like there would be more distance or physical isolation between the pins.

-rastro

Yes, a DIP-8 package will be fine too, I've just had the SOIC-8 version handy, that is all.

Cheers

Alex

[rastro]

Hello Alex,

The 617 I'm interested in LMC662A-retrofitting has the newer electrometer PCB because it has the input protection (dual) transistor(s).  I have 3 areas of concern with how well this modification would cross over from the older style electrometer PCB.

1. Different input current compensation circuitry:
-- This is likely a non-issue since the newer board probably just adds additional range.

2. Different input JFET (Q308):
-- How was it determined the Q308 is different between versions? (part number marking?)
-- Is there a meaningful difference between the new and old version of Q308?
-- Will the 18K resistors still provide the correct impedance matching for the newer Q308?

3. Addition of input protection circuits:
-- This should not be an issue since input off-set is adjusted out with R348.

Regards
-rastro

[Alex Nikitin]

Hello Alex,

The 617 I'm interested in LMC662A-retrofitting has the newer electrometer PCB because it has the input protection (dual) transistor(s).  I have 3 areas of concern with how well this modification would cross over from the older style electrometer PCB.

1. Different input current compensation circuitry:
-- This is likely a non-issue since the newer board probably just adds additional range.

2. Different input JFET (Q308):
-- How was it determined the Q308 is different between versions? (part number marking?)
-- Is there a meaningful difference between the new and old version of Q308?
-- Will the 18K resistors still provide the correct impedance matching for the newer Q308?

3. Addition of input protection circuits:
-- This should not be an issue since input off-set is adjusted out with R348.

Regards
-rastro

Non-issues on all three items, actually. I had to update the current compensation circuit anyway, newer FETs are most likely similar in parameters , just from the photos it looks like the later ones have a smaller case, and the protection transistors should not be a problem. 

Cheers

Alex

P.S. - the last measurement graph I've done today, on a fully closed and warmed-up 617:

[Vgkid]

That is pretty stable, good job.

[jitter]

Those spikes in the measurements, could they be the result of popcorn noise?

[Alex Nikitin]

Those spikes in the measurements, could they be the result of popcorn noise?

You may call it popcorn noise if you wish  . At these current level it is almost unavoidable to have some of these. Before I've closed the top cover on the 617 even some air movement may cause a spike. And even with a closed cover a small tap on the bench could cause a spike like one of those on the graph.

Cheers

Alex