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

[MiDi]

unlike a beautiful woman a triax connector is something I may actually hold in my hands in the near future!

This depends how much it is worth to you 

I noticed you used black heat-shrink. Oh-OH, some of that stuff is no good for high impedance, carbon black or some other additive makes more conductive than is desirable for this type of application. You should measure a clean piece, maybe you got lucky.

This is a point where I am pretty unsure if it matters there.
Have to confess that my experience in fA/P region is non existent 

My thoughts:
There are three possible leakage paths for heat-shrink on input connector:
-contact with input conductor: (teflon) insulated & creepage >1cm to both ends to bare conductor
-contact with earthed backplate: clearance of some mm
-contact with guarded enclosure: clearance of <1 mm or contact (e.g. due to wiggling / bending enclosure)

Last seems critical in normal mode, in guarded mode both should have same potential.
The other seem not to be that critical, if cleaned properly.
Edit:
All seem not to be that critical, if leakage paths are cleaned and held properly clean .
For last one I thought the inner enclosure is always at input voltage level, but it is always at guard Level - corrected that.

The construction seems a bit odd to me, why did they not use coaxial e.g. RG178 where shield takes the place of the sleeve?

[rastro]

To MiDi:
...
I noticed you used black heat-shrink. Oh-OH, some of that stuff is no good for high impedance, carbon black or some other additive makes more conductive than is desirable for this type of application. You should measure a clean piece, maybe you got lucky.

Perhaps you could first wrap the cable with plumbers tape which is Teflon/PTFE (probably clean it first to remove possible contaminates).  This would act as an insulation barrier to a heat shrink sleeve.

rasto

[chickenHeadKnob]

It would be simpler just to use clear heat-shrink which is cheap and available and should be  polyolefin without additives. I am always suspicious of any plastic from china as they could throw in all kinds of unhelpful additives. Some pvc is just loaded with lead, apparently it helps with the flow in molding.

[3roomlab]

relay # |

betw. contacts |

C-ES long |

C-ES short |

Hi MiDi, what does CES long and short mean?

[MiDi]

C-ES: contact to electrostatic shield
long/short: distance of the pins, there is only one ES pin between coil pin and contact pin.

coto 1240-06-2104 datasheet

[rastro]

It would be simpler just to use clear heat-shrink which is cheap and available and should be  polyolefin without additives. I am always suspicious of any plastic from china as they could throw in all kinds of unhelpful additives. Some pvc is just loaded with lead, apparently it helps with the flow in molding.

Sounds good...

[JxR]

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.

I purchased a 617 a couple weeks ago, and so far this is the only modification I have done to it.  It was definitely an improvement on the voltage output accuracy.  It varies a bit, but it is always 8mV or less from whatever is set now.

I also discovered that apparently I purchased a 617-HIQ, although it wasn't advertised as such. It has a big ass 1uF film capacitor in the analog section that I haven't seen in anyone else's pics. It is an L-revision board, and has a B5 revision firmware (I copied and uploaded the firmware to xdevs, although probably not useful to anyone).

All of my accessories are just about here (including the 100G 1% resistor), so I hope to go through the calibration procedures with it this weekend (although it isn't too far off from my initial tests). I guess I will be skipping the Columbs cal, since I don't have the procedure for the HIQ version.  Overall, it is pretty clean on the inside and seems fairly stable in the 2pA range already.

[rastro]

I haven't seen any documentation on the 617-HIQ although I have seen a few references to it on some documents on the internet. 

[JxR]

I haven't seen any documentation on the 617-HIQ although I have seen a few references to it on some documents on the internet.

Same. I have a copy of the Rev G manual everyone else does I expect, which stops at revision K on the analog board. I'm keeping an eye on eBay just in-case I see a manual pop up that is later than revision G. I don't expect to find anything, but I will certainly scan and upload it if I do.

I even bought a 1000pF +/- 0.25pF capacitor to do the coulumb cal (although I honestly would probably never use that feature).  Just figured I would do it with the rest of the cal.  It is just a ceramic though, so maybe not the right cap for the job.

[rastro]

There appears to be 3 versions of the 617-HIQ.

From an old K617 data sheet:
https://www.axitest.com/images/store/files/188931_K617.pdf

"For measurements of charge as high as 20µC, the 617-HIQ version offers three higher charge ranges. These higher ranges have proven useful on Faraday cup measurements and other static charge applications including photocopy research and development."

I wonder if the Capacitor affects the other measurement functions or just coulombs?
Is the difference among the three versions just the capacitor selection?

Another document:
https://www.nist.gov/sites/default/files/documents/2017/06/19/procedure03v430.pdf

"The charge measurements are acquired through the use of Keithley 617, 617-HiQ and 6512 electrometers. The internal capacitors of the electrometers are calibrated upon introduction into the system and any time the charge collection is suspect. Any necessary correction is applied as part of the air-kerma calculation. Five electrometers, three 6512 and two 617-HiQ, are maintained for use in the low- and high-energy ranges.  Four electrometers are maintained in the mammography range, two standard-capacitance-range electrometers (20 nC), a 617 HIQ (20 μC), and a midrange 617 (200 nC).  The HiQ is dedicated for use if a monitor chamber is required, and the midrange 617 is used for collecting charge on the Attix chamber.  The two 20 nC capacitance range electrometers are used for the customer chambers.  The procedure of calibrating a NIST reference-class chamber with each customer chamber is a quality-assurance check that rules out electrometer malfunctions."

[rastro]

It looks like the prior document also contains some calibration procedure; haven't read it closely.

"Test of high-quality electrometersNIST provides a test service (46030S) for high-quality feedback electrometers that are used in conjunction with current-type ionization chambers also being calibrated at NIST. The procedure involves electrically testing the electrometer using a feedback capacitor and computing a calibration factor, KQ.  A typical report form is found in Appendix 2.  As a check on this electrical test, the customer's current-type chamber is calibrated for one beam quality with both the NIST system and with the customer's system.  Agreement is usually within 0.2 %; if not, the calibration is reviewed and possibly repeated.Procedure for test of high-quality feedback electrometers1.  This procedure describes the calibration of a Keithley 617 electrometer, but may be applied to other electrometers if proper adjustments are made to setup and operational parameters."

[JxR]

I wonder if the Capacitor affects the other measurement functions or just coulombs?
Is the difference among the three versions just the capacitor selection?

Thanks for the research into this and documents. I will look over them soon. Currently working on finishing up my test fixture, so I can complete the calibration procedures.  Also waiting for my third triax cable to come in, which should be tomorrow.  I will be calibrating it with the aid of a 2450.

For the first question, according to the datasheet it is only the charge measurement range that is different.  I've tested it down to about 100pA so far using the SMU.  It was off by about 10pA, but that was just using the alligator clips and some banana plug cables in open air (so not ideal).  The couple of resistance tests I did looked fine, and reading voltage was fine.  The normal ranges were all available that are listed in the datasheet.

I've never seen a schematic for the 6512, but it seems that until the 6514 was released, the 617-HIQ had the highest coulumbs measurement range.  Hard to say if there is any other difference except the firmware and the film capacitor atm. Other pictures I have seen of the Revision-L board looked about the same, but we don't even have a full schematic/part list for the L boards that I know of. 

I would guess that it is probably possible to turn a 617 into the HIQ version, my firmware should be available on the ftp site on xdevs in \root\firmware\617-HIQ\...

[JxR]

The HiQ is dedicated for use if a monitor chamber is required, and the midrange 617 is used for collecting charge on the Attix chamber....

I went to through the second document and it was interesting.  They only really highlight doing calibration on the 617 using a 1nF capacitor.  But they do list the values of their NIST calibrated capacitors, their highest being 100nF.  I'm assuming this is the capacitor they use to verify the 617-HIQ.

Based on other calibration procedures for the 617, Keithley usually don't use the lowest range for the calibration procedure, but one of the mid-ranges.  For the 617, you would calibrate on the 2nC range so the use of a 1nF capacitor makes sense.  For the 617-HIQ the mid-range is 2uC.  I would venture a guess that you would normally calibrate using a 1uF capacitor.  Although, in that document they don't mention having one, so unsure which value is correct.

Digikey has, 100nF 1% Mica capacitor for ~$20 (honestly more than I care to spend)
Mouser has some 1uF 1% Film capacitors around ~$3
Both have 1% tolerance Film/Ceramic 100nF that aren't too expensive.

I'm pretty much finished with my shielded test fixture, although I still need to work on the inner box for using guarded measurements.  Also need to make some more leads to connect everything.

So far the 2450 is telling me my 100G resistor is: 100.0937G
So with the calibration current of 190pA, I'm looking at a test voltage of: ~19.0178V

Hopefully I will get to go through all the steps on Sunday.  Sounding like the wife has some plans for me tomorrow...and still need to do some additional work on my test fixture.

The 617 didn't seem to appreciate me waving around a heat gun for hours melting PTFE heat shrink.  It still hasn't calmed back down to the normal ~1-5fA yet.  Hopefully it will stabilize in a few more hours.

[Kleinstein]

I don't think a mica cap is a good idea. Mica caps have quite some dielectric absorption (slow type). They are nice at high frequency and rather stable, but not that good for slow things. So the capacitors of choice would be NP0 (ceramic), PS or PP.

With the 2450 available would could consider charge calibration from a programmed current pulse. Something like 100 nA for some 20 seconds should be more accurate than a 1% capacitor value.

[JxR]

I don't think a mica cap is a good idea. Mica caps have quite some dielectric absorption (slow type). They are nice at high frequency and rather stable, but not that good for slow things. So the capacitors of choice would be NP0 (ceramic), PS or PP.

With the 2450 available would could consider charge calibration from a programmed current pulse. Something like 100 nA for some 20 seconds should be more accurate than a 1% capacitor value.

This is great information.  Thank you for taking the time to help me out.

[JxR]

Unfortunately, calibration may have to be put off for the weekend.  My last cable that was suppose to arrive today, but is now listed as "arriving late".

I did try the calibration voltage through the 100G resistor using a BNC w/banana adapter from the SMU to my test fixture, and jumpered it to the triax input of the 617.  The 617 read 190.10pA.  Technically, this is already in spec of the +/-1.6%(rdg) + 1 count for the 200pA range.

For now, will continue to work on my test fixture and hopefully the cable will arrive by Monday.

[JxR]

Finished up most of the test fixture to help with the calibration.  The stability of the readings for the 100G resistor have increased by nearly 400% when in the guarded box.  Hopefully I can revisit the 617 calibration early next week once the cable gets in.

[MadTux]

My Keithley Firmware dumps, as requested by MiDi, including the probably latest B5 version
(Is a 7z, but can't upload that here, so rename zip to 7z to unpack)

[MiDi]

The comparison of JxRs HIQ and MadTux B5 FW revealed no differences, so the 617-HIQ uses same stock FW.
The only place left - if there is different scaling for Q-Ranges? - seems to be in NVRAM (256Bit EEPROM) U104.
I updated my unit now to B5 revision, old A3 revision and NVRAM attached and released into public domain.

[JxR]

MadTux B5 FW[/url] revealed no differences, so the 617-HIQ uses same stock FW.

That's interesting that the FW is the same.  I figured there would have been a difference for the Q setting.

I finally finished up my calibration last night (well everything except the Q cal):

V/I Resistance measurement using internal voltage source:
Measured as 100.0356G (avg of 1000+ readings from 2450)


20/200pA ranges:



200V range:


It seems to be meeting the accuracy specs in all ranges now, and doesn't seem to differ by more than 5 counts off of what the 2450 says.  I did try and source 1pA directly from 2450 to 617, but gave up on getting a stable reading.  Although it was bouncing around values that matched the 1.5%rdg + 66 counts for that range.

Overall, I'm happy with the unit.

and...
The final form of my resitance box I used for calibration. Misread the cal values from the manual and bought a 10Gohm resistor by mistake unfortunately...


For laughs, here is my monstrosity for calibrating: 10M, 1.9M, 190k, 19k (2x Caddock 1776-C621 plus a 100k/10k resistor).  One day I would like to get a nice 7 dial decade box with dials from 1 to 1Mohm, and ~10M total.

[MiDi]

That's interesting that the FW is the same.  I figured there would have been a difference for the Q setting.

It is just a scaling factor of 1k, so instead of displaying nC/uC it could simply display uC/mC instead (or corresponding scientific notation).

[JxR]

With the 2450 available would could consider charge calibration from a programmed current pulse. Something like 100 nA for some 20 seconds should be more accurate than a 1% capacitor value.

I gave this a try, but it just resulted in the 617 overflowing the calibration range.

I also tried using a 100nF film cap I measured at 100.4nF (closest I had to the supposed cal capacitor).  I placed it in the 2uC range and supplied the 19V cal voltage in my shielded box.  I calculated that Q should have been: 1.9076uC and it read about 1.9248uC.  When I tried to correct to the calculated value, it displayed "Stor" like it took the value, but original value it measured didn't changed.  I also tried just forcing the cal value to 1.9uC, and it also wrote the value, but the displayed value still was ~1.9248uC.

So, I dunno.  A GR 1409T: 0.1uC 0.05% standard capacitors seem to go for about $125 on eBay.  Maybe I will pick one up one day, but it is far from a priority.  I would certainly put the money towards a nice resistor decade box before I bought a standard capacitor.

I went ahead a re-did the voltage calibration since I think my original setup of doing it with jumpers in the test fixture was flawed.  After just using the triax-to-alligator clips on some banana-to-binding-post adapter plugged directly into the 2450, it is spot on for every single range.

So, I think I am done with calibration for now.

[MiDi]

My DIY low (tribuelectric) noise Triaxial cable made of PL75-23 and Lemo 001 101.
The Lemo cable has nearly same dimensions as Keithley SC-22, but has all teflon insulation and is much cheaper if I got it right (~20$/m vs. ~50$/m).

[JxR]

My DIY low (tribuelectric) noise Triaxial cable made...

How is your new cable working out?  Originally I planned to try and make my own cables too, but being in the US I was able to buy used 7087-TRX-10 cables for about the same price or cheaper than it would have costed me to make them.

I'm pretty much good on triax cables now, but if I ever have need for more I plan to cut some of my existing cables and use the excess I don't need to make one new cable and one shorter.

I'm seriously considering buying 7087-TRX-TBC to swap out the 2-lug bulkhead termination.  While I have a 6172 (2-lug to 3-lug) adapter, I find it a bit annoying and expect I could sale it easily for the price of the 3-lug bulkhead triax adapter.

[MiDi]

Thought it would be more complicated to build, the PL75 are very good to handle.
Tried to measure the resistance, but my crude setup was not able to give reasonable results above 100T .
Need to build proper Testbox and 2nd cable to get confidence.
This is ~100$ for 1.5m, lucky who lives in the us - low shipping costs and no import taxes...