Author Topic: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617  (Read 21236 times)

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Offline Smith

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #25 on: September 21, 2015, 04:59:03 pm »
I tried to read the main EEPROM today, but unfortunately this type (Hitachi HM4827128G-25) was not supported by my programmer. 

Today I found a problem regarding calibration: The device won't calibrate the current ranges. When I save the fresh calibration  via program select => stor the calibration values disappear, and the previous calibration settings reappear. The manual states this action should copy the settings to the NVRAM  (Xicor X2443P NOVRAM) memory. Voltage calibration does work. Anyone has experience wit this kind of issue?
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Offline rastro

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #26 on: September 22, 2015, 12:08:20 am »
I tried to read the main EEPROM today, but unfortunately this type (Hitachi HM4827128G-25) was not supported by my programmer. 

Today I found a problem regarding calibration: The device won't calibrate the current ranges. When I save the fresh calibration  via program select => stor the calibration values disappear, and the previous calibration settings reappear. The manual states this action should copy the settings to the NVRAM  (Xicor X2443P NOVRAM) memory. Voltage calibration does work. Anyone has experience wit this kind of issue?

Smith,
See page 7-6 of the 617 user manual "Keep in mind that the calibration jumper must be in the correct position, as described in paragraph 7.4.4."
It's possible that the voltage calibration may already have the same value in NVRAM as the new value you are trying to save - so it just appears you have saved a new value.

-rastro
« Last Edit: September 22, 2015, 12:22:46 am by rastro »
 

Offline Smith

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #27 on: September 22, 2015, 06:27:48 am »
I have set the jumper in the right setting. When saving calibration with the jumper on the wrong position the display says 'out' instead of 'stor'.

I am absolutely shure the voltage calibration worked as the device was far out of calibration (>1% on some settings). After adjusting the unit the voltage reading was within 1 or 2 digits of my Keithley 2000 & 6517a.

Unfortunately my programmer also doesn't support the NVRAM to make sure it writes anything. Also, the NVRAM has 256 bit for storing calibration. The device has 27 calibrated 'ranges'. So it has no more than 9,5bits for each setting, which will probably be 8 bits. Maybe I am trying to calibrate the device outside it's 'calibration working area'.

The device came in because it was out of cal on all voltage settings, some current and some resistor settings. As we don't use the coulomb measurement anywhere, none of our electrometer have calibrated coulomb settings. Maybe we should replace the damn thing, but it's only 31 years old  :D
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Offline Smith

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #28 on: September 23, 2015, 06:46:54 pm »
Complete fail! You can only calibrate 4 of the current ranges.  :palm:

Thankfully my colleague noticed. After some final adjustments the 617 now works like it should. Only thing left is the jumps every now and then. Sometimes the current jumps 30fA up and then settles down in 2pA 20pA and 2nA ranges. It took some other irradicate jumps as well, but 95% of the time there is no problem.  Calibrating the pA ranges took some time, it was difficult to find 100Gohm and 1Tohm resistors, but we did.
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Offline rastro

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #29 on: September 23, 2015, 07:25:44 pm »
Smith,

Glad to hear your cal problem is solved.  How did you instrument your 100G and 1T resistors?  Box and triax connectors for shielding?

I've only checked mine against 1G ohm.  Resistors  start getting real expensive in the higher ranges and low tempcos.  If someone was going to scrap and old electrometer it would be a good time to cannibalize a few high ohm resistors for future testing or baseline.

-rastro
 

Offline jitter

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #30 on: September 23, 2015, 07:40:04 pm »
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

The reason I asked is because with the instruments I work on, spikes like that may take it outside the specs set down by our customer. This happens with about 10% of the instruments. In almost all cases popcon noise in an AD797 is to blame.

It is a similar measurement taken on a 1k load with no voltage applied. The result shows offset and noise of which the latter must lie in a band of 4 nApp. Typical noise is around 2.5 nApp but peaks obviously take it outside the limits.
Tapping on the Faraday cage in which the electrodes, dummy load and differential amplifier are placed may indeed cause spikes.

I was wondering if the spikes in the graph of the Keithley could have a similar cause.
 

Offline Smith

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Re: Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
« Reply #31 on: September 23, 2015, 08:40:38 pm »
Glad to hear your cal problem is solved.  How did you instrument your 100G and 1T resistors?  Box and triax connectors for shielding?
The are mounted in metal boxes with BNC connectors on the in and output. We use these boxes for current calibration. I have a separate shielded case (diy) for connecting loose high resistance resistors (and other components). The box itself has an impedance of >2Petaohm.

The resistors become quite expensive indeed, the ones we use are about 50 - 100 euro a piece. It comes down to absolute value, stability and voltage dependency.
« Last Edit: September 23, 2015, 08:42:25 pm by Smith »
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Offline Alex Nikitin

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At last I've managed to build a proper triaxial cable for my 617. I may yet buy a proper low-noise one from Keithley, when I'll have £200 to spare though, as the Belden 9222 I've used is not a low-noise version and unless it is fixed in one position can produce quite a lot of charge. If it is not moving, I get the same 3-4fA input current readings (after the current settles down which takes a while) as without the cable, which is good. I've also measured the voltage source performance of the 617 - very good, with less than 1LSB (50mV) error over the complete range of +/-102.3V and the tempco about 30-40ppm/C .

Cheers

Alex
 

Offline Alex Nikitin

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A little update on the Keithley 617. A year ago I've considered <1LSB from the voltage source a pretty good performance. It is however, one area where the K617 can be improved - and there is a good reason to do it, as the voltage source error directly affects resistance measurements in the V/I mode. 25mV (1/2 LSB) error at 100V is only 0.025% but at 10V it is 0.25% and at 1V - 2.5% ! Unless you are prepared to tweak the pot at the back of the unit to set the voltage accurately at a particular point, this error makes this kind of measurement somewhat less precise. Fortunately, the performance of the voltage source can be upgraded by changing the DAC chip - the old and venerable AD7541AJN to an improved LT version - LTC7541AKN, plus changing LM308A in the x10 voltage amplifier stage to OP97. The result is a much improved accuracy (now the maximum deviation over the whole +/-102V range is less than 5mV and most of the time it is under 3mV) and also a considerably reduced LF noise at low output voltages.

Cheers

Alex
« Last Edit: January 16, 2017, 05:22:14 pm by Alex Nikitin »
 

Offline Kleinstein

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For the OP conversion I am wondering why you need the super high impedance LMC662 to replace both FETs. The feedback side is not very high impedance and could use a lower noise/drift type with no problem.

For the spike like noise, this could be cosmic or radioactive background radiation, hitting something like protecting diodes. Smaller diodes (less sensitive volume) and maybe radiation shielding could help.
 

Offline MadTux

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Anyone know where to find those black or red low leakage reed relays used on the electrometer input of the Keithley 617?

Just finished fixing a couple of broken K617s from ebay (collected over the years and never really had time fixing them ;-).
I now have 4 working ones and 1 with leaky relay, maybe someone knows where to find replacement.

It was the only one that refused to zero input offset current (R348 trimmer, always displayed something around -.1500pA). Initially started by cleaning electrometer board with IPA and deionized water (which actually fixed some of the other "broken" ones), which didn't help in this case. I then replaced the "magic" input JFET (Q308) with 2N5909 (~$5 on ebay), which actually works really great (one K617 had toasted input and got fixed that way), but unfortunately didn't solve the issue here. I then had the idea of disconnecting all unused relays (K307, K309, K310) from the input stage to isolate the leakage path. With all these relays disconnected, the K617 input offset current would zero without problems. I then simply reconnected relays until I the input offset current problem started again. Found 2 suspect relays this way, which were then removed from the board and measured for leakage using my trusty Radiometer IM6 Megaohm-Meter/leakage tester.

Good relay has a resistance >1000TOhm @ 900V, while the leaky one (K309) has around 5TOhms @ 900V, which apparently causes enough leakage so that the input offset current can't be zeroed any more.

BTW, the most important thing when zeroing the  input offset current is time, especially after the board was cleaned. I found that it sometimes takes a day or so until all moisture has disappeared. A hair dryer is also helpful. The +/- .0015pA  offset current mentioned in the manual are IMO also a bit optimistic. After a good warmup I easily could adjust them to +/- .0050pA, but adjusting them to within +/- .0015pA required constant fiddling on R348.
« Last Edit: August 04, 2017, 08:57:04 pm by MadTux »
 

Offline math_indy

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I could not find COTO 1240-0197 anywhere (aka Keithley RL-70) however I found COTO 7301-05-1000 relays (>1TOhm) in stock at Digi-key which may do if you cannot find anything else.  I "dead bugged" the new zeroing relay into the spot where the leaky relay was.

Here is a video showing the leaky relay that was replaced:


It would be nice to have some of the original relays AND a specification sheet for the original relays.  I can't find a specification sheet for 1240-0197 anywhere?

After replacing the relay it worked much better but getting full performance required cleaning BOTH circuit boards.  I got some laboratory grade methanol from eBay and used a small airbrush gun to blast the methanol onto those boards.  For safety reasons do that outside because of the flammable methanol and associated fumes that you don't want to inhale.

Does anyone know exactly how the zero-correct works?  Is it any different from suppress?  It appears that both zero-correct and suppress are executed by software.  I think this especially true because I cannot find any analog sample-and-hold so it seems just a software correction-no?
 
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Offline rastro

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Tom,
Great video and follow-up.  Great troubleshooting techniques - I'll have to remember this work around.  Hope the subtitue relay works out.  Let us know.  Also if you are going to scrap the old relay it would be interesting to open it up to see how it's built.

-rastro
 

Offline MadTux

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I'm quite busy lately, so I somehow forgot to write on how I fixed it.

Anyway, these red relays are quite easy to fix. Just pull out the old reed switch tube with some pliers and put a new reed  inside the relay coil.

The real magic lies in selecting a good reed switch. Forget the cheap crap from china, only a few GOhm of isolation resistance (because they have no sulfur hexaflouride filling). They will also start to arc at relatively low voltages (300-500V, usually). The original reed switches inside the red relays aren't that great either. (in terms of arcover and isolation resistance). For replacement, I finally found some afordable Harmlin MARR5 reed switches that are rated to an isolation voltage of 1000V and an isolation resistance of at least 1TOhm and fit inside the old relay coils.

 Real isolation resistance is more like 50-100TOhm @ 1000V, just at the limit where I can measure reliably. Also couldn't measure breakdown voltage, because my isolation tester/megaohmmeter only goes up to 1000V.
http://www.ebay.com/itm/Hamlin-MARR-5-Reed-Switch-SPST-NO-1-3-A-1000-VDC-10-W-10pcs-1-Lot/122199268347?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m2749.l2649
http://m.littelfuse.com/~/media/electronics/datasheets/reed_switches/littelfuse_reed_switches_marr_5_datasheet.pdf.pdf

The only problem with these is that they won't reliably switch at 5V supplied by the 74HC04. So you need to overvolt the 74HC04 by applying something around 8V from the output of the first preregulator stage. (from emitter of Q309, placed some ceramic caps in there for good measure, too). I initially hacked an addon board so I can use 15V from C305 directly, but I got an noisy instrument afterwards, probably because of capacitivly coupled rectified AC noise. The it occurred to me that I might just overvolt the 74HC04 with some 8V from the 1th preregulator output, which worked.

Leaky black relays are a bit more difficult to fix, because the reed switch is epoxied inside. An option might be to drill it out. But I destroyed mine in the process, so I ended up replacing them all with Harmlin MARR5 reed switches and some reed relay coils I found on ebay. Doesn't look nice, but works just as good (maybe not as good as the red relay hack, because these coils have shielding inside) The coils are held in place by some copper wire and an eyelet soldered to the PCB, btw.
« Last Edit: October 03, 2017, 10:38:23 pm by MadTux »
 

Offline rastro

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Hello MadTux;

Interesting information.  The 3 pictures you just posted look like different PCB's.  I looks like one has 3 of of the series transistors for the HV supply replaced with transistors that have different pin-out due to the crossed legs.

-rastro
 

Offline MadTux

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Yes, 2 different K617, both bought broken for little money on ebay over the course of a couple of years. The one with funny electrometer drive stage transistors was especially broken, because the previous owner (dubious dealer of all sorts of test gear on ebay  ;-) had no clue on how to fix it and replaced parts at random, apparently without success. Finally sold 2 rather broken K617 as a package for a good price, which I bought. Because I couln't find the original MMPS U60 for a good price, I replaced them by MJD350, which has different pin configuration, IIRC.
« Last Edit: October 03, 2017, 11:41:03 pm by MadTux »
 

Offline razberik

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math_indy: I see some problems in your approach. It is not very clear in your video, but it seems that you are touching these very sensitive areas with your fingers. Perhaps you know what are you doing and you also say that in video.
But anyway for everybody: Never touch picoamp parts with bare fingers. Of course I break this rule sometimes. ;D But I touch only leads, since there is no change in resistivity. Sensitive are insulation areas, like hi-ohm resistor bodies. When I manipulate with these parts, I use tweezers. When risk of touching, gloves are must.

I don't think methanol cleaning is the best idea when I am not absolutely sure what exactly the problem is.
Please read this topic, there are valuable information:
https://www.eevblog.com/forum/testgear/keithley-64856487-teardown/
There is also a schematic of input of this instruments. Since it is descent of K617, the principle is the same, ZeroCheck included.

Also that 1240-0197 COTO relay is a special shielded relay ! You definitely cannot replace it with 7301-05-1000. It can perhaps work in DC area since it has good insulation, but transient response would suffer. That guard shield helps condition material between contacts and outer world to zero voltage difference, so it wouldnt suffer stabilizing time.
Explained here: https://www.embedded.com/print/4375459

Modern replacement should be 1203-0147. Unfortunately no datasheet nor information on COTO website. I emailed czech and german representative yesterday about this relay since I am also interested in it! No answer yet.
1203-0147 is used in Keithley 6514 and 6517A.

I would use MadTux approach. Pull out old reed and replace it with new one. Dont screw guard foil. Perhaps Standex-Meder 1A66 reed should work too:
http://cz.mouser.com/ProductDetail/MEDER-electronic-Standex/KSK-1A66-1015/?qs=KFo7JewZbUHhbHIVxk%2FaJw%3D%3D
It seems that reed is covered with polyetylene (third best insulator after sapphire and teflon) and wrapped in shield foil. Therefore I would wrap new reed with foil and then try to insert it into coil package.

AD) Also, It is a luck this bad relay was discovered since it is really bad. But if relay has some small leakage problem, it wouldnt be discovered in nanoAmp range and unshielded arrangement.
« Last Edit: October 06, 2017, 01:17:59 pm by razberik »
 
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Offline rastro

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This is a really interesting discussion on the reed relays.  It seems like faulty range-relays could be a hidden/frustrating problem on many of these electrometers as they age.

It would be great to find an comparable substitute for these reed switches.  One that could be inserted in the original solenoid.  Ideally a part that has the same AMPERE-TURNs (AT) turn-on level as the original parts.  This would eliminate the need for modification to the solenoid supply.  However it seems like reed switches with higher breakdown voltages also require higher activation current through the solenoid.  I've outline some of the considerations but these parameters still need to be evaluated.  I think determining the AT value of the original reed/solenoid would be good start.

* Reed Switch Requirements

** AMPERE-TURNs (AT):
   The product of the number of turns in an electromagnetic coil
   winding and the current in amperes passing through the
   winding. Used to quantify reed switch operate and release
   sensitivities.

** Breakdown Voltage:
   Higer voltage tolereance requires higher AT due to larger gaps
   required between reeds.  What is the design requirement of the
   current reed switches?

** Hermetically Sealed:
   Is this required for high impedence?

** Form:
   SPST n.o. (Normally Open)

** Carry Current:
   This sould be fairly low???

** Insulation Resistance:
   What leakage can we measure on known working switches?

 

Offline Kleinstein

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If needed one could improve the sensitivity a little by adding a magnetic return path on top of the relay. It does not need to go all the way to the contacts and thus would not cause much extra leakage. Since the magnetic circuit is not closed, the gap at the contacts will not be that important for the sensitivity. 

Sensitive reed relay usually com in a ferromagnetic case. The case is not only a a shield for the outside, but also to improve sensitivity by a magnetic return path.

The separate reed contacts are usually just glass - these special relays use a plastics (e.g. polystyrene) cover directly around the glass. The direct contact might be important to prevent surface leakage. Usually the glass itself is a good enough isolator, but unless very dry, there can be a surface layer of water that can cause leakage.  Putting a new glass in the old plastic tube might not be enough and could be difficult. I have not seen the details, but my guess would be using a kind of heat shrinking (e.g stretching it first) on the polystyrene.
 

Offline razberik

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Does it happen to you too ? You want to buy your shielded relay, you open your favourit supplier, start browsing relay section and then ... you can't find shielded relays section !  :rant:

I have few old obsolete Meder HI12-1A79 which were replaced by 1A85.
I sacrificed one and tried to install shield foil. I wasn't able to install copper foil, it simply wasn't possible. I used aluminium foil and that wasn't easy to install though.
It was really difficult to arrange the foil not to touch reed switch. Gloves wearing is absolutely must.
See photos. I didn't have time to test it yet, but I am going to work on it.

The separate reed contacts are usually just glass - these special relays use a plastics (e.g. polystyrene) cover directly around the glass. The direct contact might be important to prevent surface leakage. Usually the glass itself is a good enough isolator, but unless very dry, there can be a surface layer of water that can cause leakage.  Putting a new glass in the old plastic tube might not be enough and could be difficult. I have not seen the details, but my guess would be using a kind of heat shrinking (e.g stretching it first) on the polystyrene.
This seems to be plausible. I firstly thought that the only reason copper foil is wrapped around plastic encapsulation is only manufacturing process. But I didn't think of you mentioned.
 

Offline rastro

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razberik/Kleinstein;

I'm not sure what you mean by "sensitivity".  Does this mean lowering the amount of current required to activate/close the reed switch?
So are you saying that adding foil around the reed switch will make it activate easier?

Is the ferromagnetic casing focusing magnetic flux toward the reed switch?  Would this be similar to a torroid focusing the flux on a coil.??
 

Offline razberik

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Kleistein was perhaps talking about this (see picture).
 

Offline rastro

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Kleistein was perhaps talking about this (see picture).
I'm not sure what the picture is supposed to convey.  At this point I'll just assume that 'sensitivity' is the current (VA?) required to activate the reed switch.
-rastro
 

Offline Alex Nikitin

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Reading this discussion about relays I've remembered that I have a quantity of encapsulated miniature reed relays - REMtech MRFL80005, 5V operation, internal screen for 50 Ohm line impedance connection, sub-ms operating time. Just out of curiosity I've measured the resistance between open contact pins (everything else disconnected, relay suspended in the air inside a metal box I use to measure high resistance with my Keithley 617). Using V/I resistance mode I've measured about 2.2 PetaOhm = 2200 Tohm at 100V, both polarities applied (about 45fA leakage), after ~30min settling time for each polarity. I did not even clean it, just soldered straight out of the bag. Not bad for a chance eBay purchase several years ago. I use these relay for audio with good results, but it looks like they are useful for high impedance stuff too (up to 125V as that is the contact rating) !

Cheers

Alex

P.S. - tried with the relay internal screen connected to ground - the leakage at 100V is around 10fA (measured resistance ~10 Peta Ohm, with the idle current of the setup, including the K617 around 0fA (+/-2fA fluctuations, with the voltage source at 0V).



« Last Edit: October 11, 2017, 09:43:27 pm by Alex Nikitin »
 

Offline rastro

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I think you also want to check leakage from the reed contact pins to the solenoid connections also.
 


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