Keithley 2001 owners: request for a simple experiment - Page 1
Forum section: electronics / testgear

author: openloop
author_rank: Regular Contributor
timestamp: July 28, 2020, 04:06:08 am
content: Maybe an old firmware thing, but...If you have a K2001 and it is not too much of a trouble...Set your Keithley to 10 NLPC: config->DCV->speed->hiaccuracy[EDIT:] Set range to "manual" 20VSet a power supply to about 18VMeasure it with the Keithley. [EDIT:] The shorter leads the better.Now, do this quickly: manually flip polarity of the connection. I simply pull dual banana out of the power supply and insert it backwards.What is the reading on the Keithley? As expected?Do it several times if it is.Please tell me what happens, your firmware version, line frequency at your place (50, 60Hz, or 400) and a model (like "M" or not)Thanks!

author: DuPe
author_rank: Regular Contributor
timestamp: July 28, 2020, 08:52:06 am
content: hi openloop,here: Rev. B15, A01, Mem 2, No warm up+ 18.0014..- 18.0014..+ 18.0018..- 18.0016..+ 18.0019What is your observation?Cheers PeterEdit: 50 Hz

author: openloop
author_rank: Regular Contributor
timestamp: July 28, 2020, 10:22:45 am
content: DuPe, Thank you !!!That's not what I expected, strangely enough.(I'll deter disclosing my observations for now - to keep the experiment clean)Maybe it is indeed my old firmware/hardware (A08) glitching, or me living in a 60Hz land...Can you repeat the experiment, but with as short leads as possible?Set range to "manual" 20V <-- I forgot about this.Plus sweep a range of voltages: from 13V to 19V in steps of 1V? No need to write readings down, as long as there is no glitch (You'll know when you see it).And asymmetric: e.g. +17V --> -6V, if equipment allows.Thank you!

author: DuPe
author_rank: Regular Contributor
timestamp: July 28, 2020, 11:01:45 am
content: Hi openloopwires are 1m long each, can't make them much shorter witout rearranging my bench. I repeated the experiment reversing wires at 10V to 18V: Nothing unusual. Also with assymmetric voltages (e.g. +18V --> -7V) no glitch. Really keen to see solution of this puzzle.Cheers Peter

author: openloop
author_rank: Regular Contributor
timestamp: July 28, 2020, 11:14:19 am
content: Thank you!In a couple of days I'll report (here) what I've found.For now I'll just wait a few more days to see if somebody else corroborates my observations.If nobody other than me sees "The Glitch" then there's little value to my findings (but I'll post them anyway)Thank you again!

author: DuPe
author_rank: Regular Contributor
timestamp: July 28, 2020, 11:18:14 am
content: Just for completenes

author: sokoloff
author_rank: Super Contributor
timestamp: July 28, 2020, 11:29:02 am
content: Any need for measurements on a K2015?

author: openloop
author_rank: Regular Contributor
timestamp: July 28, 2020, 11:30:10 am
content: DuPe,Thanks for the photos.In your pictures, range is in "auto".I forgot in the first message to mention that it needs to be in manual. Sorry for that.Can you quickly re-do the experiment?I'm sorry!

author: openloop
author_rank: Regular Contributor
timestamp: July 28, 2020, 11:33:18 am
content: Quotesokoloff, thank you!I doubt that the same bug is in there too. It might be in 2002 though...You can try of course...

author: DuPe
author_rank: Regular Contributor
timestamp: July 28, 2020, 11:36:13 am
content: Wellcome openloopchanging to fixed range does not change anything (i.e. brave old 2001 shows no issue)CheersPeter

author: openloop
author_rank: Regular Contributor
timestamp: July 28, 2020, 11:40:06 am
content: QuoteThanks!A bummer for me though.

author: openloop
author_rank: Regular Contributor
timestamp: July 28, 2020, 09:19:42 pm
content: Common guys! It takes just a few seconds to run this.I need to replicate the effect, especially if your instrument is an older (single ROM) model.Pleeeease...

author: coromonadalix
author_rank: Super Contributor
timestamp: July 29, 2020, 02:32:14 pm
content: Please be patient

author: Kjelt
author_rank: Super Contributor
timestamp: July 29, 2020, 06:01:32 pm
content: I let my wife video the swap because it took too much concentration to do the fast swap.But my old A05 K2001 is so slow I get intermediate readings of 3,3V to -7V before settling probably because the integration time in high accuracy position is very slow ?Proof

author: openloop
author_rank: Regular Contributor
timestamp: July 29, 2020, 10:53:08 pm
content: Kjelt,Thank you!That's not integration time...

author: rodpp
author_rank: Frequent Contributor
timestamp: July 30, 2020, 01:43:05 am
content: Quote from: openloop on July 28, 2020, 11:33:18 amThe line here is 60Hz, filter off. It seems ok.

author: openloop
author_rank: Regular Contributor
timestamp: July 30, 2020, 02:49:32 am
content: rodpp, thanks!Good to know about 2002. Otherwise imagine the wrath of certain people around here.

author: bson
author_rank: Unknown
timestamp: July 30, 2020, 03:42:23 am
content: B17, A02+18.0005...-18.0007...+18.0005...-18.0007...+18.0004...-18.0006...+18.0005...-18.0005...60HzFilter offEnough warm up for it to stop drifting noticeably (5min?)

author: bson
author_rank: Unknown
timestamp: July 30, 2020, 04:05:13 am
content: Actually, looks a little better after a 30min warmup, but essentially it's the same - no glitches or oddities. The DP832 PSU was calibrated with my 34465A.

author: openloop
author_rank: Regular Contributor
timestamp: July 30, 2020, 05:16:52 am
content: bson, thanks!OK, time to fess up.Context: Just for fun I'm playing with and modding K2001's ADC. To learn about finer points of ADCing and attempt to reduce Keithley's noise in the process.K2001 is very conveniently designed for messing with it.Among many other very cool things, I've stumbled upon the following:When converting analog signal into digital form, this ADC uses two counters. One for counting the difference in the number of up and down slopes during dual slope portion of the cycle, another is simply a clock count for the duration of the final slope.And, apparently, due to some major screw-up, designers come one bit short for the dual slope counter if integration time is too long (e.g. 10 NPLC). 16bits are not enough!Thus, this counter goes through integer overflow!On my device (A08, single ROM), in 60Hz country, overflow happens around +- 7.5 to 8V at ADC, which translates into +- 15V at the front panel (12.5V in 50Hz zone?).When I realized that, my first instinct (for some reason) was try to fool my Keithley by switching overflowed voltages so very fast that the micro wouldn't be able to catch the wrap around.And it WORKED!Overflow, basically, maps two different voltages to the same value on ADC's output. For example, looking at the raw readings (on my K2001) +13.7V and -17.3V produce the same integer value!Why the same trick doesn't work on later models? I do not know.Several theories:1) They fixed the hardware - the big chip sitting on the ADC board. That's expensive. Unlikely.2) They (hardware designers) added an RC filter that slows the transition enough for the microcontroller to see through the ruse.3) They added like 0.1NPLC peek to check which ballpark they're landing at.4) You guys suck. And your power supplies suck too.Long leads and small, high ESR caps might not be able to fully recharge Keithley's internal circuits in under 160ms required to trick the micro. I don't recommend switching at the multimeter end - might tarnish the contacts.So, here you have it. Small mystery solved. This is FUN!