Do I have exceptional Keithley 2000 or poor Keithley 2001? - Page 1
Forum section: electronics / testgear

author: mimmus78
author_rank: Unknown
timestamp: October 22, 2016, 07:27:28 pm
content: Hi guys,I'm running some monitoring of my LM399 reference based board. I'm doing this by one Keithley 2000 and one Keithley 2001 multimeter but I have unexpected results.I was expected the Keithley 2001 to be far less nosier than the Keithley 2000, but this is not the case.I'm using the same configuration for both meters:10 NLPC10 averaged readingsautozero sync10 V range for K2000 - 20V for K2001This K2000 is really stable, never expected it to be as noise free. Results don't change too much even at 10 NPLC without averaging.Attached readings graph (ignore fist hour and more, meters where still cold) ...So my question is do I have exceptional Keithley 2000 or do I have to go hunting inside Keithley 2001 for some problem?Anyway now I think I really need one 3458A ...

author: Kleinstein
author_rank: Super Contributor
timestamp: October 22, 2016, 08:13:55 pm
content: Both curves still show quite some noise. So for me it is more like a rather noisy K2001. To check for the noise source one could do a simple test with shorted inputs, to separate in noise of the input-stage / ADC and that of the meter internal reference. You have 3 lm399 references involved: the DUT, and one in the K2000 and K2001 each.Besides true noise there is also possible RF interference, that might look like noise but can influence the meters to a different degree.

author: Andreas
author_rank: Super Contributor
timestamp: October 22, 2016, 09:41:40 pm
content: Hello,looks plausible to me.My K2000 has around 7uVpp or around 1uV std deviation when measuring a 7V LTZ1000 reference in 10V range.When I bought my K2000 the seller had hooked it together with his K2001 to a SMU.Noise on K2001 was significantly higher.With best regardsAndreasEdit: has the averaging an effect when reading out by interface?My measurements are with 10 NPLC and no averaging.

author: mimmus78
author_rank: Unknown
timestamp: October 22, 2016, 10:08:49 pm
content: Well I cannot exclude RF noise, but to me it's not the case.I run also some tests with a battery and got consistent noise figure similar to that of the reference board.With shorted leads at 20V range I got:12uV p2p noise in 1 minute at 1 NPLC (no averaging)7uV p2p noise in 1 minute at 10 NPLC (no averaging)From what I read in the "noise test" thread this is ok for a K2001 ... but maybe I misinterpreted it.Starting thinking that K2000 is fair beast for 10V measurements.Just thinking to sell my K2001 to someone that needs all those special features it has and get a more modest and new 34465A with fresh calibration.

author: saturnin
author_rank: Regular Contributor
timestamp: October 22, 2016, 10:31:37 pm
content: Hi,I have exactly the same experience. Surprisingly, K2001 is noisier than K2000. It is confirmed by their specifications (see attached screenshots from user manuals):K2000 10V range: rms noise at 1NPLC is < 4uV (based my experience, it is lower actually), whileK2001 20V range: rms noise at 1NPLC is 6 uV typical (0.3ppm of 20V is 6 uV)I think it is not so well known fact and I was personally quite disapointed when I found out...

author: mimmus78
author_rank: Unknown
timestamp: October 22, 2016, 10:47:50 pm
content: Quote from: Andreas on October 22, 2016, 09:41:40 pmYes but not as I expected ... fast test, run for only 10 minutes and a total of 47 readings.STDEV with AVG = 3.22 E-06STDEV with NOAVG = 3.40 E-06seems to little difference.PS: time was different for the two tests but for the purpose of producing the chart I put only one timeline

author: mimmus78
author_rank: Unknown
timestamp: October 22, 2016, 10:56:56 pm
content: Quote from: saturnin on October 22, 2016, 10:31:37 pmohh I searched this noise specs in K2001 manual and for some reason I din't catch up ... double disappointment

author: VintageNut
author_rank: Frequent Contributor
timestamp: October 22, 2016, 11:19:05 pm
content: My K2000 is stable and quiet measuring my 2X Fluke 731B. My K2000 was calibrated by Keithley and the data shows 1ppm deviation from 10V. I have adjusted both of my Fluke 731B to match the 10V of my K2000.I am also borrowing a DMM7510 and it is much better than the K2000. I can see the 731B drift 1.5 ppm over the course of the day and then the 731B returns to its morning value the next day.This drift of the 731B is below the resolution of the K2000. As good as the K2000 is, the DMM7510 is much better. The DMM7510 measures a few ppm lower than the K2000 but that is not important to me.

author: Andreas
author_rank: Super Contributor
timestamp: October 23, 2016, 04:19:25 am
content: Quote from: mimmus78 on October 22, 2016, 10:47:50 pmOk that is no difference. (only statistical stray).With averaging 10 readings the STDDEV should go down by a factor of 3.I guess that the averaging only works for the display on the instrument.With best regardsAndreas

author: martinr33
author_rank: Frequent Contributor
timestamp: October 23, 2016, 05:46:50 am
content: I have a 2001, 2010, 2002 sitting on my bench right now.I measured a LTZ1000 unbuffered reference, 100 readings, NPLC 10, digital filter. Noise is the SD reading, only maybe 10 mins warmup so there mightbe some drift.2001: 5uV noise 7th digit2010: 1.5uV noise 7th digit2002: 1uV noise 7th and 8th digitsthe 2010 is a bit of a surprise. It is a stripped down machine with a segment display.Also - on the 2000, 1uV of noise on the 10V range is below the sixth digit, which represents 10uV increments. So maybe there's a bit of quantizing going on, chopping off some of the noise.I see the same kind of thing with my 6.5 digit 34401A.

author: mimmus78
author_rank: Unknown
timestamp: October 23, 2016, 10:06:15 am
content: I 2010 is very similar to 2000, I think it has new firmware ad improved better selected references.Anyway stdev of my K2000 seems way better than 1uV ... that's nice.This things that AVG is not working is strange ... meter really slow down when enabling averaging.Maybe we should read better the manual and read the averaged vale with some other command.

author: mimmus78
author_rank: Unknown
timestamp: October 23, 2016, 11:57:00 am
content: Well I used "READ?" to read measurement.This command even if slow down measurements according to AVG setting just returns last measurement and makes no AVG.

author: TiN
author_rank: Super Contributor
timestamp: October 23, 2016, 04:35:14 pm
content: Time for some bit of RTFM2001/2002 has two modes for averaging, moving and repetitive.Default is moving, with amount of samples = 10.Also READ? commands resets triggering and math systems. FETCH? with proper sample window setting would be better solution, and maybe provide less noise (I did not test this myself though).I can try it in next few days, if you guys want.

author: mimmus78
author_rank: Unknown
timestamp: October 23, 2016, 07:13:06 pm
content: Anyway I don't understand why "READ?" slow down with AVG enable if it just return one reading.I tried half a dozen solutions but I never managed to make it working, some examples just uses GPIBfunctions my library seems not to implement, and understanding all the details to avoid those specialcommands and settings need some more RTFM.Last attempt was to read 10 or 100 samples and do AVG by myself, but neither this worked and gotonly an empty reply.TiN if you have some "spare" time to check this it will be helpful, if not I will try it on next weekendand let you all know :-)========================================================================# Setup Keithley 2001l.ibwrt(ud, "*RST")l.ibwrt(ud, ":SYST:AZER:TYPE SYNC")l.ibwrt(ud, ":SYST:LSYN:STAT ON")l.ibwrt(ud, ":SENS:FUNC 'VOLT:DC'")l.ibwrt(ud, ":SENS:VOLT:DC:DIG 9; NPLC 1; AVER:COUN 1; TCON REP")l.ibwrt(ud, ":SENS:VOLT:DC:AVER:STAT OFF")l.ibwrt(ud, ":DISP:WIND:TEXT:DATA \" \";STAT ON;")l.ibwrt(ud, ":DISP:WIND2:TEXT:DATA \" \";STAT ON;")l.ibwrt(ud, ":FORM:DATA ASCII")l.ibwrt(ud, ":TRAC:POIN 10")l.ibwrt(ud, ":TRIG:COUNT 1")l.ibwrt(ud, ":TRIG:DELAY 0.000")l.ibwrt(ud, ":TRIG:SOURCE EXT")l.ibwrt(ud, ":SAMPLE:COUNT 10")l.ibwrt(ud, ":TRACE:CLEAR")l.ibwrt(ud, ":INIT")#time.sleep(20)l.ibwrt(ud, ":TRAC:DATA?")

author: martinr33
author_rank: Frequent Contributor
timestamp: October 23, 2016, 07:26:44 pm
content: Turns out that the 2001 has a couple of settings in the setup menu.Autozero - should set to synchronous. Autozeros every reading.Power line sync - should also be synchronous for high precision.Once I set these two parameters, I lost triggering - the K2001 defaults to high speed measurements, not high precision. Now, the K2001 is down at 1.57uV SD, measuring a 7.11Vhttps://goo.gl/photos/X81xs1nazapSfSMN8Not sure how this will translate into GPIB commands. But it seems like this is static setup.My 6.5 digit HP 34401A shows 7.11374, with the occasional flicker to 5. But I tweaked it to be closer to the 2001. The 2002 is notionally in cal.

author: TiN
author_rank: Super Contributor
timestamp: October 24, 2016, 04:40:51 am
content: Stacking 200x's like that is enough to get 2002 out of cal though..I have day off tomorrow, so will run the test. I'm done with fairy-tale LTZ module, so can use it as reference.Here's initial reference data as base. Reference is in DIY TEC thermal chamber, so there is no TC other than DMMs.RAW DSV data. First hour or so had temperature setting change, that part removed from graph below.Here's code used for sampling on 2002 (2001 is exactly same):QuoteNo need for any special commands, just writes and reads.

author: mimmus78
author_rank: Unknown
timestamp: October 24, 2016, 12:57:45 pm
content: Quote from: martinr33 on October 23, 2016, 07:26:44 pmMeaning GPIB=========================================Autozero synchronous = ":SYST:AZER:TYPE SYNC"Power line sync = ":SYST:LSYN:STAT ON"High precision = ":SENS:VOLT:DC:NPLC 10"I used "NPLC 1" to get more ADC noise and hopefully accentuated averaging effect ...

author: mimmus78
author_rank: Unknown
timestamp: October 24, 2016, 01:21:39 pm
content: >> Stacking 200x's like that is enough to get 2002 out of cal though..Unless you use some audiophile grade oxygen free copper OFC super compensated leads and some black insulation box for power supply.Calculated stddev to almost 1uV@10 NPLC for the 3458A, like in spec ... 10X better than 2000 and way better than 2001.

author: doktor pyta
author_rank: Frequent Contributor
timestamp: November 01, 2016, 12:47:29 pm
content: I also have problems with K2001. With 1NPLC and 100 AVERs it has much higher noise than HP34401A set for 100NPLC.Have anyone tried to make upgrade similar to TiN (https://xdevs.com/article/k2002_nadc/). Does it make sense in case of K2001.PS. Are there official schematics of K2002 available?

author: lukier
author_rank: Unknown
timestamp: November 01, 2016, 01:00:35 pm
content: Quote from: doktor pyta on November 01, 2016, 12:47:29 pmThat would be interesting. I remember from the K2001 thread that TiN once tried replacing LM399 with LTZ1000, but it made things worseQuote from: doktor pyta on November 01, 2016, 12:47:29 pmIn general I don't think so, but TiN did some rev-eng of the ADC board AFAIR.

author: saturnin
author_rank: Regular Contributor
timestamp: November 01, 2016, 05:39:19 pm
content: I was also thinking of replacing LM399 with LTZ1000 in K2001. Unfortunately, you simply can't desolder LM399 and connect +7V ref from LTZ1000. It is because 2mA current is fed to LM399 from -10V ref source located in the A/D board. Moreover, the negative reference terminal of LM399 is connected to a virtual ground created by U328 (AD707).A possible solution is shown in attached drawing. It is not a complete design - it rather shows necessary modifications in the original reference circuitry to support LTZ1000. I ceased to implement it when I realized that main contributor to K2001's noise performance is not the voltage reference, but rather the A/D converter...

author: mimmus78
author_rank: Unknown
timestamp: November 03, 2016, 08:23:48 pm
content: Yes K2000 is 2 or 3 times less noiser than K2001 ... maybe K2000 can be a nice project, but I don't think you will get noticeable improvement on noise.You would improve the 10V accuracy but the other ranges will continue to be as good as a 6.5 digits multimeter.

author: Zucca
author_rank: Unknown
timestamp: November 04, 2016, 08:42:36 am
content: Quote from: saturnin on November 01, 2016, 05:39:19 pmWow! I got the same idea.TiNinspired me with by doing that in a HP 3245A (see articlehere). It this case things are much better than in a K2001, see below how the LM399 is used there:In a K2001 the LM399 has no reference to ground...shall we discuss this in a new thread? Oh let´s see... I think some Voltnut Dragon will destroy our idea pretty soon. It THEY didn`t manage the upgrade yet, there will be a good reason behind.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 04, 2016, 09:40:41 am
content: Before thinking about an update of the reference one should check where the noise is really coming from.There are 3 main sources for noise:1) the input amplifier2) the ADC circuit itself (might depend on voltage and integration time)3) the referenceSo the first step would be a careful characterization to see where to start from. Using different integration times also helps to separate sources, as there can be 1/f type noise.Measuring noise at short circuit takes out the reference for most of the part. Low ranges are often mainly the input amplifier. Knowing the circuit one can usually get a good estimate on how much noise in the 20 V range comes from the amplifier.Measuring noise with a low noise voltage adds the LF noise of the reference.p.s. I just looked at the available diagram for the 2001: it looks like they have two references inside: The LM399 for long time stability (and ohms source) and a second one (VR801 = 6.4 V zener) as a working reference for the ADC, that is one the ADC board itself.

author: Zucca
author_rank: Unknown
timestamp: November 04, 2016, 11:06:19 am
content: Interesting...Quote from: Kleinstein on November 04, 2016, 09:40:41 amQuotehttp://www.datasheetarchive.com/dlmain/2645466039e35e26f019f39d2feadc007b3f14/M/1N4571AEDIT:So did some homework... this is the VREF generation circuit on the ADC K2001:On R845 there is the voltage produced by VR801, let´s call it VD... the current flowing in R845 must go through R843 and R847... so it meansVref+=VD/2K*1.2K+VD=1.6*VDVref-=-VD/2K*3.2K=-1.6*VDSo Vref(VR801) is about 10.24V.... In theory we can replace the VR801(VD=6.4V) with a LTZ1000 (let´s say 7.15V). And by changing R843 with 900R and R847 with a 2,9KR we should get a new Vref (LTZ1000) at about 10.36V, rock stable.Ah didn´t check any DC current, heater stuff yet... just want to point out the mod looks possible here...Just don´t know how much this mod will impact the performance...

author: Kleinstein
author_rank: Super Contributor
timestamp: November 04, 2016, 01:57:00 pm
content: If reference noise is a problem, one could replace the 1N4579 with one of those low noise Chinese 2DW232. They are even about the same voltage, just the ref. current might need adjustment via R864 to get the current for low TC. There is a thread one this ref. in the metrology section.A first test could be just to measure the noise of the used reference. Maybe this type of zener is also very low in noise - at least it uses a higher current than the LM399, so the potential is there. Though a good reference, I have not found anything about it's noise specs.The long time reference should still be the LM399 reference, so it would not make much sense to use an expensive LTZ1000 here. Using a separate short time and long time reference would make sense if the short time one is lower noise - than it can be a good solution, as one would need an internal calibration / adjustment from time to time anyway to compensate for drift in the integrating resistors and amplification to bring it to 10,x V.The reference contributes to the overall noise in two ways. First is the obvious, that the measured value is proportional to external voltage divides by ref voltage. So relative noise level of the reference will also appear at the output. Extra time used for AZ and zero phase might amplify the noise a little. This can be a significant part at higher numbers. As this is low frequency noise, it is hard to impossible to improve by filtering.With many of the ADCs (also the K2001) there will be a second effect, as for zero reading they use +ref und -ref to compensate in a kind of PWM style. So noise from that modulation frequency band (e.g. kHz range) can also contribute, especially for near zero readings. Here filtering from C821 might just starts to be effective. So a larger capacitance here could also help a little to do some more filtering. The LM399 is specified at about 100 nV/Sqrt(Hz) in the kHz range - so it could be a significant contribution (e.g. more than most other amplifiers). However with a significant voltage to read, the LF noise should be the more important part, especially with slower readings.

author: Zucca
author_rank: Unknown
timestamp: November 04, 2016, 02:38:28 pm
content: Quote from: saturnin on November 01, 2016, 05:39:19 pmI almost missed this point, so by improving the Vref ADC we also improve the LM399 Zener current! Nice! BTW The K2000 has the LM399 Zener just hooked at the +14V and GND ... mmmm...Quote from: Kleinstein on November 04, 2016, 09:40:41 amThanksKleinsteinI am learing a lot from your knowledge... finding out where the noise is coming from is the most important task. I agree with you. Time to do some tests.

author: plesa
author_rank: Frequent Contributor
timestamp: November 04, 2016, 06:04:29 pm
content: Quote from: Kleinstein on November 04, 2016, 01:57:00 pmThats a relly good idea. I do not have problem with noise on K2001, but on electrometer 6517, which use the same ADC board it can be valuable upgrade.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: September 04, 2025, 10:33:28 pm
content: related to K2001Quote from: Kleinstein on November 04, 2016, 09:40:41 amI'm right about ? ... if Vref around 10V , so zener current 2ma ? in this schematics :https://www.eevblog.com/forum/testgear/do-i-have-exceptional-keithley-2000-or-poor-keithley-2001/msg1063517/#msg1063517So anyway , i made experiment on this zener , basically short it by 2x47uf 100V polypropylene caps , adding 2x33uf on each opamp 15V , and 20 ohm R , basically some power RC -filter ...and nothing changes ... i wish it changes something ... but it seems placebo ..Guessing adc zener can be be crossed out (with some but ) ...with averaging 100 ( not a digital ... ) noise almost eliminated ( i look at 20V range) and what left some wierdo slow fluctuation on microvolts ... when measurement slowly drifting one side to another ( i would say 40 sec period ) ... and that P-P drifting around 8uV. and that not by connected ref ... same slow drifting i see on shorted 20V range . Thinking it by design ..... some kind residual formed by complex array of switches to make complex measurement signal .... Switches kinda mediocre to the latest one .... Is it worth to replace some ? there is no SOIC anymore for good one .Now "but" on zener : my guess .... after mods , the noise became more uniform that gives a better filtration , I probably add some couple 20uF ceramic as Zen. shunt . seems no need to worry for leakage. LT1097 - 0.5-1 uV noise wonder if it true, i have ADA4522 , worth to replace ?

author: Kleinstein
author_rank: Super Contributor
timestamp: September 05, 2025, 05:23:43 am
content: A slow periodic part can be beating between mains frequency and sampling rate. This would than change over days / time of day as the mains frequency can change a little.Another source are thermal fluctuations / oscillations that could be in the 1/10 to 1/100 Hz range.The ADA4522 is usually not a good replacement for a LT1097. The AZ amplifier adds possible higher frequency internference and the ADA4522 has quite high current noise, while the LT1097 is made for rel. high impedance circuits.The CMOS switches for the signal rooting usually don't add noise. If at all they would add leakage current, which is not that critical in most places. For the switches the specs are often limited by production testing and the actual performce is usually much better than the spec limits.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: September 05, 2025, 07:34:01 pm
content: Thanks Kleinstein ,thoughtful ..haven't able to hook data reading ... to show stats .. need later ...last mod: same red 47uF as 399 shunt:this is around 100 sec Filter ave 100 , slidingMIN\MAX 3 uV ... as soon as made a picture ... it start climbing up ,I bet resulting output , it a complex algorithm , including internal temp (LM75 onboard) , digital offset, voltage on both zeners, uncertainties in ADC ... weather on Mars and something else ...I even don't know a precision of float value CPU calculating inside , rounding can add somethingand roll up or down completely random , can sit for a while then go up a few numbers , then immediately back ... or siting on the same # for 10-15 sec ...swing it on side ... bam voltage change to 4-5 microvolt , 399 affected ... - so it always referred to 399i probably add some polymer caps there and there ... but i guess it doesn't cure much ...UPD: Forgot to say: i have another 2001M , both connected to the same 10V source, and marching in opposite directions. It not a source deviation. 2010 as more primitive ,do not have such behavior and do not indicate 10 microvolt source bouncing.