Keithley 192 - last digits jumping around - Page 1
Forum section: electronics / testgear

author: e61_phil
author_rank: Frequent Contributor
timestamp: October 26, 2013, 10:30:23 am
content: Hello,my problem with my Keithley 192 is a very unstable reading. For example I feed stable 10V (source: Hioki 7005; measured with my HP3478A) in and the display jumps between 9.98V and 10.01V.This problems occurs with any voltage in any range of the multimeter. I've tried to disconnect the 1910 AC Unit and inject a stable DC Voltage into the multiplexer input for the 1910 signal. This results also in an unstable reading.I've checked the 5V analog, 5V digital, +15V, -15V, +30V and -30V all voltages are looking fine.I've checked the LM399 output -> very stableI've checked the +20V reference voltage -> also very stableIn the service manual (http://bal4u.dip.jp/hobby/pdf/Keithley-3546.pdf) on page 48 (Fig. 7-14) the signal of the integrator is shown. I've attached the measured signal of the integration capacitor. I have no experience with such charge balancing single slope converters. Does this signal looks good?Or any other ideas whats going wrong with my Keithley 192?ThanksPhilipp

author: bernroth
author_rank: Regular Contributor
timestamp: October 26, 2013, 05:03:57 pm
content: Have you tried to measure a single cell battery already? Use short wires to connect the battery to your meter.Sent from my iPhone using Tapatalk -now Free

author: e61_phil
author_rank: Frequent Contributor
timestamp: October 26, 2013, 06:48:19 pm
content: The input voltage is stable. I've measured the input voltage with my HP3478A, I've also tried a short and a resistor in the Ohms range. Everthing results in an unstable reading.Another problem is the service program. I've read PRG 8 initializes a self test. But PRG 8 results only "Pro 8" on the display and nothing else happens.

author: macboy
author_rank: Super Contributor
timestamp: October 27, 2013, 01:34:18 pm
content: Quote from: e61_phil on October 26, 2013, 06:48:19 pmDid you press enter? Try PRGM, 8, ENT.

author: e61_phil
author_rank: Frequent Contributor
timestamp: October 27, 2013, 02:18:34 pm
content: I've tried it now. But nothings happens.For the other programs (like 6.5digit mode), the enter key isn't necessary.

author: e61_phil
author_rank: Frequent Contributor
timestamp: October 27, 2013, 09:22:36 pm
content: Does anyone here in the Forum have an Keithley 192 with software version C4? (Shown on bootup)I don't know what I'm doing wrong to enter program 8

author: Unknown
author_rank: Unknown
timestamp: October 27, 2013, 09:27:30 pm
content: I remember seeing the same behavior with the non-functional program 8 in a 192 that is still on my 'to fix' pile. Since the meter appeared to work fine otherwise I assumed it was a firmware bug. I didn't try executing the self test via GPIB.

author: e61_phil
author_rank: Frequent Contributor
timestamp: October 27, 2013, 09:31:13 pm
content: That's a pity, because test 1-4 could be very usefull to find my "noise" sourceThx

author: grumpydoc
author_rank: Super Contributor
timestamp: October 27, 2013, 09:42:43 pm
content: QuoteHave you checked ripple as well as just voltage?

author: e61_phil
author_rank: Frequent Contributor
timestamp: October 27, 2013, 09:59:11 pm
content: Yes, the ripple voltage is about 20mVpp. It is rather noise than 50Hz ripple. I think that is ok?I've also checked the ESR of the supply caps. The caps are looking fine.The "noise" on the display is depending on which range the Keithley is. The described 9,98 to 10,01 was seen in the 20V range. In 2000V range the display jumps between 9 and 12 volts with stable 10V input.

author: wiss
author_rank: Frequent Contributor
timestamp: December 31, 2013, 11:13:24 am
content: And the saga continuesI have noticed a ringing with a period of about 1 us and a ring-frequency of 30 - 100 MHz. The attached pictures are of the integrator output.The first shows the ring and the left part of the second is the end of the single slope phase which has quite some distribution in time, I think.From the circuit diagrams there seems to be a general lack of decoupling caps, the only one close to the integrator is from digital 5 V to signal ground! Why not Digital ground?Any ideas?

author: wiss
author_rank: Frequent Contributor
timestamp: December 31, 2013, 11:38:26 am
content: Ok, measured the output from the integrator with trig on the output of the single-slope comparator, maximum deviation is a few (1 MHz) counts, this can not be the problem.

author: wiss
author_rank: Frequent Contributor
timestamp: January 06, 2014, 10:00:07 pm
content: Some progress, today I wanted to check if the range-fets actually switches the gain of the input. They do. But then I realized that I should try it in resistance-mode. Rock stable! The attached scope-images was done with a slightly more modern scope that I borrowed and are taken while in ohms with the inputs shorted and display at stable .000001. The integrator output looks horrible! How can this be so stableWell, it works ...

author: codeboy2k
author_rank: Super Contributor
timestamp: January 07, 2014, 01:52:13 pm
content: Quote from: wiss on January 06, 2014, 10:00:07 pmThose integrator spikes are fine, they occur when Q347 turns on and off. The op-amp is too slow to react and the switch on or switch off at Q347 allows the compliance voltage from the balancing current source to blow through to the integrator output( at switch on time ), until the opamp can catch up and bring it under control. Switch off at the bottom is similar, the CB current source goes away and one end of the integrating capacitor is dragged down hard by the signal reference ground until the op amp can catch up to correct it. However those spikes don't affect the charge balancing phase whatsoever; it's the counts that matter.Did you previously say the ohms was unstable? now it's stable?Quote from: e61_phil on October 26, 2013, 06:48:19 pmIf the ohms is stable and a shorted input is stable (on ohms??), I'd say the ADC is working fine. Are all ohms ranges stable when making a measurement? If so, it's clearly not the ADC or anything else after that. Perhaps you need to look only at the MUX FET Q328, or the protection FETS Q329 and Q330.

author: wiss
author_rank: Frequent Contributor
timestamp: January 07, 2014, 02:16:23 pm
content: It is stable at approx zero for shorted input in all ohms-ranges, I did not try with a finite resistor but will tonight.Prime suspects right now are the protectiondiodesjfets on the dc-input.Replacements for those old high impedance jfets are only rated 30V, but on this input they need to take up to 55 VAnyway I will check the leakage of some bc54x in EB-C diode mode and maybe that will work as good.

author: wiss
author_rank: Frequent Contributor
timestamp: January 07, 2014, 02:18:21 pm
content: Regarding the ranges, I probably confused previous owners "all voltage ranges" with "all ranges" or something

author: wiss
author_rank: Frequent Contributor
timestamp: January 07, 2014, 05:57:41 pm
content: Wrong! When in ohms the instability is related to the measured value!4-terminal:10 ohms: .2k all stable, 2k digit 4 unstable (? almost out of range)100 ohms: .2k digit 4 unstable, 2k digit 5 unstable (of value, not range), 20k digit 4 unstable1k: 2k digit 4 unstable, 20k digit 5 unstable (? almost out of range), 200k all stable.1M: 2000k digit 4 unstable, 20M digit 4 unstableWhen in DCV the instability is related to range, not value!

author: wiss
author_rank: Frequent Contributor
timestamp: January 07, 2014, 07:45:13 pm
content: When in 2k ohms mode, 1k on Ohm-hi - Ohm-lo, reading 999.xxx (x=noise).Plug in -1 V on sense-terminals: reading 505.xxx.20k ohms mode (rest same as above) : reading 0.9998x (x = +-2 digits)Plug in -1V on sense: reading 2.74x ( x = +-2 digits)

author: wiss
author_rank: Frequent Contributor
timestamp: January 12, 2014, 07:39:14 pm
content: I've been looking into the input-mux today.The jfets used are speced to 30 V reverse gate voltage and in this circuit they can get up to 50 V! Or, well, there is this 1 Meg gate-resistor on all of them so if they go avalanche at 30 V the gate current will be 20 uA. This current has to come from somewhere which I guess will be which ever has the higher voltage of drain or source. This could very well be the drain which sits on the very high impedance buffer input???I measure a typical drop over the gate resistors of 2.5 to 3 V so a few uA of gate-current.Can this circuit possibly work at all?

author: wiss
author_rank: Frequent Contributor
timestamp: January 12, 2014, 09:35:57 pm
content: Ok, the 2-3V pretty much matches the voltage drop over 1 meg when a 10 meg scope-probe is attatched...less than 1 mV when measured properly...

author: wiss
author_rank: Frequent Contributor
timestamp: January 15, 2014, 06:38:22 pm
content: A realization: If the instrument is stable within +-2 digits (or so) at 0 display the number of CB-cycles has to be the same and the single-slope phase has to start from a level that is within 1/500 (or so) of maximum integrator voltage (max about 1500 counts). At zero input it should not be possible to see a difference between the "input" and "zero" integration-curves of a oscope!While taking the attached image the instrument was in ohms-range displaying a 0 reading (Ohm-hi shorted to Ohm-lo and Ohm-sense-hi to Ohm-sense-lo).The difference between Ohm-sense-hi (the left one) and Ohm-sense-lo (the one fully visible) is several hundred.

author: wiss
author_rank: Frequent Contributor
timestamp: January 15, 2014, 07:50:00 pm
content: Shorting the R382 (input of integrator to ground) the output of the integrator goes up to ~0.5 V and stay flat until single-slope, all phases the same. Integrator is not leaking or noisy.Shorting output of bufferamp to ground(after reading data-sheet of LM343) => Display readings all over the place and integrator looks like during zero input!Problem in Transconductance-amp!

author: wiss
author_rank: Frequent Contributor
timestamp: January 20, 2014, 08:40:00 pm
content: I found it.The resistor-network R369 is used by the Charge Dispenser to generate 5V, this 5V is more like 5.45 and jumping around up to 0.1% (pin 6 of R369 and pin 3 of U316). When I put a 330 uF cap from pin 3 of U316 to ground the flickering decreased dramatically and is now dependant on range, it is not visible on the 2 kV range. Seems like the custom Caddock networks not age well...

author: codeboy2k
author_rank: Super Contributor
timestamp: January 21, 2014, 06:52:27 am
content: Aha... you persevered!! I've been following your updates. Thanks for posting as you went along.So really?? it was all voltage reference instability??This goes to show you (and everyone playing along at home) that it's really important to focus on voltages and references up front.Your work here has taught me that I should also first look at and characterize voltages and references to ensure they are spot on before looking elsewhere.If you need to, that 2v / 20v reference can be improved by todays standards. At least the opamp and voltage dividers can be updated to some fairly accurate parts these days (a good DC opamp and a good monolithic matched resistor network on a chip).

author: wiss
author_rank: Frequent Contributor
timestamp: January 21, 2014, 07:34:01 am
content: Regarding the references, 2 V, 20 V and -10 V are perfect! The 20 V main divider (in the feedback to makes the lm399 into 20 V) is in the same network as the problematic one is stable to 6 digits when measured out of the circuit. It was the +5 V only used for the current source that was the problem. I will also check the other Caddock network on the board, I think that there still is a minor problem somewhere.In the process I replaced U316 and U308 with new lf356's, this resulted in a large offset reading at about 8000 counts on the .2 range, I guess that they are too fast and pick up the 30 MHz ringing or something...How can I upgrade the dividers and opamps?Can I get networks/dividers from other sources than Keithley?What opamp can improve the circuit? lf356 does not improve performance.

author: codeboy2k
author_rank: Super Contributor
timestamp: January 21, 2014, 12:42:59 pm
content: I misinterpreted it.. Since R369 and U316 are also part of the 2V/20V references, I thought your problem was there.If the reference is perfect, don't touch themAs for that other half of R369 and U316, I don't have a part in mind, but once you figure out what the constant currents should be there then I am sure you can find a good DC op amp to match it or better. There are plenty of suppliers of matched resistors too. The most important parameter there is probably the tempco matching.

author: wiss
author_rank: Frequent Contributor
timestamp: January 21, 2014, 12:57:18 pm
content: R369 has values of 50k, 150k, 922k, 11.8k, 68.1k, 33.2k and 7.14k...It is the 50 and 150k that is the problem (that I know of) and now they do not have to be temp-matched!So my current plan is to amputate pin 6 of R369 and build my own 150 over 50k divider there, I am measuring the new divider now and with a 10u cap it seems pretty noise-free.If I replace opamps I can not use devices that are faster than the lf351, right? (They have to be dead at 30 MHz) Or I will have to get fast ones and slow them down with a feedback cap? Then there`s no need to replace the lf351`s in the first place.

author: codeboy2k
author_rank: Super Contributor
timestamp: January 21, 2014, 01:38:37 pm
content: I would think you wouldn't want to pick something too fast. A too fast part can cause oscillations if the rest of the circuit isn't designed for it. You seem to be aware of this already.I'd use the parametric search of the manufacturer's websites and find a FET input opamp with 50pA bias currents or less, can handle the supply voltage range in the circuit, can slew fast enough (> 11V/us , say up to 100V/us) and has GBW between 30MHz and 50Mhz. A part like that should be ok there.But why are you trying to change the op amp? I only suggested it before in case the reference was out and you needed to rebuild it.The lf356s have very large input offsets, in the mV's . I suspect after you replaced them the new ones had a different input offset than the older ones, and so the meter needed to be re-calibrated to zero out the effect of those offsets.Even if you replace them again with a part searched parametrically, they will still have different offsets and the meter will still need to be re-calibrated on all the ranges.

author: wiss
author_rank: Frequent Contributor
timestamp: January 21, 2014, 03:30:21 pm
content: I suspected the opamps to be the problem, lf351 is out of production and lf356 which I had a few of is the recommended replacement. Any offset after the MUX will not produce a display-offset, there is no zero-calibration, a non-zero display indicates a major fault.

author: wiss
author_rank: Frequent Contributor
timestamp: January 21, 2014, 06:37:23 pm
content: Transplant performed!

author: wiss
author_rank: Frequent Contributor
timestamp: January 21, 2014, 07:27:52 pm
content: Reasembly:It has a permanent 1 mV offset on low DCV ranges and 10 mV on the high.It is way off in calibration.ACV offset is 6 mV on .2 and 2, 60 mV on 20, .6 V on 200 and 6 V on 2000.kOhm offset is less 3 than 10 counts on all ranges (6.5 digit mode)

author: codeboy2k
author_rank: Super Contributor
timestamp: January 22, 2014, 12:55:13 am
content: looks good !man.. I need one192's and 199's always look nice on a bench

author: wiss
author_rank: Frequent Contributor
timestamp: January 22, 2014, 08:49:19 am
content: I`m not that impressed with it, I would not recommend it. It seems to be hard to get spare parts, there are loads of interference all over the analog circuitry and it does not have that fast conversion-rate. Even thou the integration-time is 100 ms in 6.5d-mode it will do 4 integrations per measurement and maybe 2 updates per second. The prema 6001 is far better.

author: codeboy2k
author_rank: Super Contributor
timestamp: January 22, 2014, 01:10:59 pm
content: Quote from: wiss on January 22, 2014, 08:49:19 amI like the Prema's too, but they are hard to find over on this continent. I think they are more prevalent in the EU.

author: wiss
author_rank: Frequent Contributor
timestamp: January 22, 2014, 02:08:49 pm
content: Yea, I`ve seen a few 6001 go for around 200 - 300 eur on ebay.de the last few months.

author: sync
author_rank: Frequent Contributor
timestamp: January 22, 2014, 02:40:30 pm
content: Doesn't the Prema 6001 needs 1s integration time for 6.5 digits?And a HP 34401A does only 2.4 reading/s in 6.5 digit mode too (50Hz mains). So the 2 reading/s of the Keithley 192 is not too bad.

author: wiss
author_rank: Frequent Contributor
timestamp: January 22, 2014, 02:48:42 pm
content: Yes, it is not a bad reading-frequency but in relationship to the complexity it seems not worth it.But then then keithley is probably a few years older than the prema.From what I understand the prema do integrate the input the full 1s and not just a fraction of the time, the keithley integrates the input less than 1/4`th of the conversion-time.

author: sync
author_rank: Frequent Contributor
timestamp: January 22, 2014, 03:20:22 pm
content: The Keithley do four measurements per reading: input, zero, reference, zero. So it can compensates drifts in the analog front end and ADC.

author: wiss
author_rank: Frequent Contributor
timestamp: March 10, 2014, 08:37:52 pm
content: Not finished yet!The instrument still show a large offset, about 600 counts and gets noisy at higher input (relative range).By looking at the integration-traces I noticed that, with input shorted, all integrations end at about constant level except the reference integration which jumps back and forth at least one half CB-step. The LM399 is 6 digit stable.When applying ~1.9 V in, the input integration starts to jump in the same way. (and maybe the zero-integrations)(2V range) When shorting the input with a 10Meg resistor the reading is ~0.26... V, using a 1Meg gives ~0.026 V on the low ranges. (28 nA of bias current)Is the AD542 damaged?It is out of production?Or is the bootstrap amp misbehaving?

author: wiss
author_rank: Frequent Contributor
timestamp: March 10, 2014, 09:15:11 pm
content: As it turns out, when I had my magnifier-lamp over the instrument the offset increased and decreased when I removed it. At wetting the AD542 the offset drops a few counts and when heating it with a finger the offset increases. This should not be the case, right?

author: SArepairman
author_rank: Frequent Contributor
timestamp: March 11, 2014, 06:16:12 am
content: a dc offset caused by RF is caused by RFI rectification.The technique to manage it is to shield the inputs of the circuit, using ferrite and LC/RC PASSIVE filters.The RFI is rectified in semiconductor parts (i.e. the input of the differential amplifier) and causes a DC offset.A balanced choke might be a one part solution, put at the input of your differential amplifier. Though I think it gets rectified everywhere as it gets into your system and causes anomalous behavior everywhere, you need to figure out what to shield.You will notice some better multimeters have the input go through a big ferrite ring, probobly for this reason.http://www.analog.com/static/imported-files/application_notes/AN-671.pdfhttp://www.analog.com/static/imported-files/tutorials/MT-096.pdf

author: macboy
author_rank: Super Contributor
timestamp: March 11, 2014, 12:53:15 pm
content: Quote from: wiss on January 21, 2014, 07:27:52 pmSmall offset in a DC range is can be caused by something as simple as a temperature difference between the calibration temperature and your room's temperature. On a meter capable of resolving less than 1 ppm of full scale, this is expected as normal. Use the Null/Zero key to null this out. The DCV accuracy specifications often state "After Nulling" or something to that effect.AC offset is also expected as normal. This is caused by noise in the RMS converter (and other circuitry) and will not affect the reading when the measured value is about 10x as high as the noise (the input signal and the noise add as root-sum-of-squares). On the AC ranges, the specifications of the instrument will always include a disclaimer about low level accuracy. For example, Keithley 199 specifies AC accuracy at >2000 count (at 5.5 digit which is 303000 count full scale). The Agilent 34401A specifies AC accuracy down to only 5% of full scale, with reduced accuracy from 1% to 5% and no specification below 1%.Resistance offset is also expected since the test leads have resistance, and because it is fundamentally a voltage measurement that is affected by voltage offsets just as voltage measurements are. Using the Null/Zero key is common practice with resistance measurements. If you see a negative resistance when the leads are shorted, then you really have a meter out of calibration

author: wiss
author_rank: Frequent Contributor
timestamp: March 11, 2014, 01:05:56 pm
content: Seebeck-coef of Cu-SnPb can be up to 3 uV/k, 60 mV would be a tempdiff of 20 kK, no, I do not have that much heat.Cu-Kovar up to 40 uV, tempdiff of 1 kK, still quite hot.60 uV would be reasonable, 60 mV not.

author: macboy
author_rank: Super Contributor
timestamp: March 11, 2014, 03:01:16 pm
content: Quote from: wiss on March 11, 2014, 01:05:56 pmYou said 1 mV DC offset (low ranges) and 10 mV on the high ranges, not 60 mV. Even 1 mV is probably on the high end of what to expect, but you can and should null it out with the Zero button.You said 60 mV on AC measurement. This is not due to thermoelectrics, this is (mostly) noise in the RMS converter and other front-end circuitry. It is present on even the best meters, since even just 1 ppm (20 counts on a 6.5 digit meter) represents noise of -120 dB. We have a complex RMS conveter circuit with ~1MHz (or better) bandwidth and 1 Mohm input impedance. Therewillbe noise.

author: wiss
author_rank: Frequent Contributor
timestamp: March 11, 2014, 03:22:25 pm
content: Ok, maybe confusing a bit, not at home now, anyway, 0.28 V off at 1Meg input "short" is way to high, AD542 has a rated bias of 25 pA, should give 25 uV (still not good for this kind of instrument, the AD542 is supposed to be "selected")Thermal error should be less than some 10 uV after warmup.I do not really care about the AC-unit now.Anyway, a 600 count error on the 2 V range is out of adjustment-span.

author: wiss
author_rank: Frequent Contributor
timestamp: March 11, 2014, 05:49:06 pm
content: Turns out that Prog 8 do work after all!Just no display feedback and I only tried in 5.5d-mode.When measuring the output from the buffer amp (U303) in REF-mode, a few digits of noise on the output of the amp, about the same as displayed on the instrument. In 0-mode: low noise and some offset. In DCV-in mode and 1.9 V in: the same noise as in REF-mode.In DCV-in mode with 1Meg on the input large offset and some noise.

author: wiss
author_rank: Frequent Contributor
timestamp: March 11, 2014, 07:58:22 pm
content: After removing Q331, Q332 and replacing U301: no changeWhat could I use instead of LM343 for U303?

author: wiss
author_rank: Frequent Contributor
timestamp: March 11, 2014, 09:56:06 pm
content: R338, a 1Meg in series with a MUX-jfet gate (for AC-zero) has ~24 mV over it, all other gate resistors have some 10 uV over them!JFET-dectomy!

author: wiss
author_rank: Frequent Contributor
timestamp: March 11, 2014, 10:20:54 pm
content: Yup! Less than 10 pA bias-current now.And I don't really need that non-rms AC-converter anyway...

author: r6502
author_rank: Regular Contributor
timestamp: March 22, 2019, 01:07:23 pm
content: Hello All,I knew, this is an old post, but have also bought a 192 instrument, that has some similar problems.When the instrument came to me, it does not show any stable reading. The following actions I performed until now:I found out, that the electrolytic capacitors in the power supply where at the end of the lifetime and I changed all of them.Then I found out, that the +30V supply was nearly at 45V, caused by a defective resistor R127, 5.36k, in the feedback of the LM317, so I changed this resistor, and +30V is ok nowThe Reverence voltage was drifting for more than 3mV during warm up. May be this was caused by the defective 30V supply at about 45V, that was exposed to the heating connection of the reference. Reference was exchanged and the Vref 6,95 V is fine now and does not drift any more.Reading is much more stable now.Now my question:The 20V reference voltage, generated out of the LM399, is still drifting during warm up of the device. Could it be, that the feet back resistors around U313 are defective? The 20V ref was measured: 19,8734V (cold) 19,8741V (warm), so about 0.7mV drift during warm up. This was measured with a 199.I think, this drift is a little too much - am I right?Kind regards Guido

author: Kleinstein
author_rank: Super Contributor
timestamp: March 22, 2019, 01:46:17 pm
content: The 20 V reference is not that important. The important internal reference is +2 V.A +45 V supply (instead of 30 V) could have cause some damage to OPs or similar. The OPs used in this old instrument are also not that low in drift as one expect it today. So some drift during warm up can be normal.

author: coromonadalix
author_rank: Super Contributor
timestamp: March 22, 2019, 02:30:31 pm
content: they write in the manual to wait at least 1 hour prior use