Author Topic: Restoration glory of Keithley 2001 DMM  (Read 212423 times)

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Online Kleinstein

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Re: Restoration glory of Keithley 2001 DMM
« Reply #650 on: November 12, 2016, 09:19:01 am »
The max326 likely has a different pinout, but there is an mx4052 as an direct, better specs  replacement for the cheap CD4052 / MC14052.
 

Offline Zucca

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Re: Restoration glory of Keithley 2001 DMM
« Reply #651 on: November 12, 2016, 12:55:19 pm »
I'll be opening one of 2001s soon, could try some stuff!  >:D

Can't wait to read about Doctor TiN taking care of his K2001 patient!
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Offline TiN

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Re: Restoration glory of Keithley 2001 DMM
« Reply #652 on: November 12, 2016, 12:57:28 pm »
All four my patients are actually healthy. I just got lured by that LTZ talk...
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Offline mimmus78

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Re: Restoration glory of Keithley 2001 DMM
« Reply #653 on: November 12, 2016, 02:20:05 pm »
Kleinstein: maybe max4052a?

Don't know Motorola is still more flat/linear on Ron vs Vcom and temperature variance.

Because value of this Ron must be calibrated out I speculate this factor maybe important to be as constant as possible or you may introduce more noise in the measurement.

« Last Edit: November 12, 2016, 02:27:00 pm by mimmus78 »
 

Offline lukier

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Re: Restoration glory of Keithley 2001 DMM
« Reply #654 on: November 12, 2016, 02:23:55 pm »
Another weekend, so more measurements of K2001 for reference. This time input resistance on 20V and 200mV ranges.

I've used the voltage divider method with this 1 GOhm resistor in series with a TTI DC linear PSU:
http://uk.farnell.com/ohmite/sm104031007fe/resistor-10kv-1-gig-1/dp/1550776

Measured as: 996.2889 MOhm. Meter set to DC, 10 NPLC, AZ, LINESYNC. I've used two voltage measurements, one random instantaneous reading and a mean of 100 samples in a buffer (but K2001 reports that with limited precision).
V1 = voltage from the PSU, V2 with 1 GOhm resistor in series to form a voltage divider with the multimeter.

RangeV1 [V]V2 [V]R [GOhm]
20V10.0025189.989635772
20V10.00009.99471878
200mV0.199932020.1791151028.572
200mV0.199900.179368.699

I'm a bit worried about the 200mV range, as it is slightly under 10 GOhms and Keithley specifies > 10 GOhm on all low ranges. Maybe this is due to my poor measurement setup (normal banana test leads, although I've cleaned the resistor with isopropanol before taking measurements).

In the next episode I want to run TiN's DMM noise tests, but for that I need to prepare my GPIB setup and rewrite EZGPIB scripts into Python + linux-gpib.
 

Online Kleinstein

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Re: Restoration glory of Keithley 2001 DMM
« Reply #655 on: November 12, 2016, 04:31:22 pm »
The max4052 is specified at much lower R_On, 60 Ohms compared to around 600 Ohms for the MC14052. Thus resistance variations are also much lower. Still the specified leakage is lower. It is just the max4952 is a dedicated high quality switch, whereas the MOS4052 is a cheap part from the logic part family. So the price is something like 10 times as high). Getting a dip version might be difficult though.

For the input resistance, one should treat the input current more like a bias current that is slightly voltage dependent. So even with no external voltage one would see an input current. Depending on the individual instrument and temperature the current can be different. The relationship is often also nonlinear: like low current in the +-15 V range and increasing close the the ends of the range.

Having a 20 mV drop on the 1 G resistor is an input bias current in the 20 pA range - this a good value.
 
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Offline plesa

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Re: Restoration glory of Keithley 2001 DMM
« Reply #656 on: November 18, 2016, 09:52:38 pm »
If you want new VFD for Keithley 200x/24xx/26xx/700x send me PM (price will be about $100).
 

Offline mimmus78

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Re: Restoration glory of Keithley 2001 DMM
« Reply #657 on: November 19, 2016, 11:33:23 pm »
It seems I finished repair of my K2001, ohm ranges are now all "working" even if now calibration of ohm ranges is upset.

At the end I replaced the Motorola MC14052 with same part from ON (datasheet was exact copy of the Motorola one).
With this new part I have no more leakages at any range and self test passed without any error.
I tested all high ranges up to G ohm with a 10M resistor and all ranges seems few counts from each other.

I compared ohm measurement noise of this meter with the K2000 and I noticed something strange to me.
I know this meter is more nosier than K2000, but comparing both meters in the same condition my K2001 is much noisier that the cheaper brother.

I'm testing both meters in the same conditions:

 - 1M ohm metal film resistor stick directly in multimeter by using TiN method (cotton buds)
 - 10 NPLC
 - no filter
 - range 2M (1M for K2000)
 - 2 hours meter warm up
 - 10 minutes wait time after you stick the resistor to the meter

K2000 stabilise immediately and all measured values are at maximum of 1 PPM apart (1 count).
K2001 have average jumps of 5PPM and occasionally it jumps of 10PPM (50 to 100 counts).

Does it looks normal? Maybe we need to send current source op amp into retirement too.

Anyway the strange thing is that the lower ranges are much better. For example 10K resistor at 20K range has only 3 to 5 digit jumps (that is 0.5 ppm).
This is the same ppm noise specified for the 10V range, that is also as best as you can go.
Also if you measure low ohm (10K or 100K) values in high ohm ranges noise calm down.
 

Offline 2N3055

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Re: Restoration glory of Keithley 2001 DMM
« Reply #658 on: November 19, 2016, 11:43:33 pm »
ON Semi  IS Motorola... It is spin of old Motorola semiconductor division... Processor division was spun off as Freescale...

Good to hear it works now!!

K2001 being noisier than K2000 seems to be a fact.. I don't remember details, search for a topic exactly about that here on forum..
It seems to be normal behaviour, or at least common behaviour.. I think you might contribute to discussion there..

Buona notte Mimmi!!
 

Offline mimmus78

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Re: Restoration glory of Keithley 2001 DMM
« Reply #659 on: November 19, 2016, 11:59:27 pm »
Quote from: 2N3055 on Today at 10:43:33

K2001 being noisier than K2000 seems to be a fact.. I don't remember details, search for a topic exactly about that here on forum..
It seems to be normal behaviour, or at least common behaviour.. I think you might contribute to discussion there..
I know it's noisier I also made a direct comparison of the twos (check below) but still seems too much in this case.
I will appreciate if someone can make a test in the same conditions. I have to decide if to send it for calibration or continue the troubleshooting ...














« Last Edit: November 20, 2016, 12:09:42 am by mimmus78 »
 

Online Kleinstein

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Re: Restoration glory of Keithley 2001 DMM
« Reply #660 on: November 20, 2016, 12:38:55 pm »
For comparison I would do the measurements without filter - there can be different types of filters and one might end up getting unfiltered number via GPIB while filtered data are shown.

The Ohms ranges of the K2001 might have a slight design flaw: using the LM399 for the current source, but the zener reference for the ADC. The Ohms circuit also is quite complicated and I don't really understand it, but it does not look really low noise with quite high resistors, and not that much voltage at the current setting resistors. It looks more like made to also provide the very high resistance ranges, but compromising on the rest. So a relatively high noise level for the Ohms ranges might be normal. It could also be different for the low range like 1 K and high ranges like 1 M.

It would really make sense to have comparison data, before trying to fix something that is not broken, but just a poor design.
 

Offline mimmus78

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Re: Restoration glory of Keithley 2001 DMM
« Reply #661 on: November 20, 2016, 01:13:50 pm »


As I get "normal" noise with low value resistors also on high ohm ranges  I think this is normal ... So I will not touch again unless I'm sure there is really a problem.

The real surprise is how better is K2000  vs K2001 on noise. Wondering if LTZ1000 experiment will be more appropriate with this meter than K2001.

Change reference, change ohm circuit reference resistors, change current shunts (AC to DC chip is the same) and you will have a hell of multimeter.

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Offline TiN

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Re: Restoration glory of Keithley 2001 DMM
« Reply #662 on: November 20, 2016, 04:49:24 pm »
I have opening for "free" sampling hours next week, as will be doing other stuff, and can plonk 2002, few 2001s and 3458 to measure same batch resistors over next days for comparison.

Just tell me ranges you want to compare and settings. NPLC 10, no filter, 4W with AZ I assume?
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Offline mimmus78

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Re: Restoration glory of Keithley 2001 DMM
« Reply #663 on: November 21, 2016, 09:59:18 am »
Hi TiN I made those checks:

  • 1M 1/4W 1% metal film resistor - 2M 2W range - 10 NPLC - no averaging - offset comp. OFF
  • 1M 1/4W 1% metal film resistor  - 20M 2W range - 10 NPLC - no averaging - offset comp. OFF
  • 10M 1/2W 5% carbon resistor - 200M 2W range - 10 NPLC - no averaging - offset comp. OFF
All test run for 30 minutes at least, sampling as fast as I can (1 sample every 2 seconds circa with this settings), meter was on for some hours.

Workflow:
  • Read measurement from GPIB and data saved on csv.
  • Calculated std.dev with 50 samples window.
  • Produced chart.

I think it will be useful to investigate also voltage drop noise on resistor with oscilloscope during measurement.
I kinda find something weird yesterday when I was checking resistor voltage drop with multimeter but didn't have time to investigate if is measurement process artefact or if is really noise.

I've attached here charts and ods files with raw data (open office spreadsheet file format).
I noticed that once you look at noise in ppm of range it became not that bad ...
« Last Edit: November 21, 2016, 08:30:58 pm by mimmus78 »
 
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Offline TiN

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Re: Restoration glory of Keithley 2001 DMM
« Reply #664 on: November 23, 2016, 04:13:51 am »
I don't have enough 1M resistors to do so, so let's start with DCV first to set the minimum noise level floor.

Added some datalog data over meters (2x2001, 2x2002, 3458) to compare into DMM noise thread. Here's the link.

Results so far:

3458A : <1.2 uV noise ~0.12 ppm/range
2002-4 : <2.5 uV noise ~0.125 ppm/range
2002-6 : <2.5 uV noise ~0.125 ppm/range
2001-20 : <15 uV noise ~0.75 ppm/range
2001-21 : <22 uV noise ~1.1 ppm/range
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Offline TiN

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Re: Restoration glory of Keithley 2001 DMM
« Reply #665 on: November 24, 2016, 05:03:58 am »


2001-22 at the end of the graph. Obviously it's sick with noise >3ppm/range and wild jumps.

One thing I was thinking about A/D noise of 2001, perhaps one can try using lower noise zener on ADC board. For example chinese 2DW232?  ;)
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Offline Zucca

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Re: Restoration glory of Keithley 2001 DMM
« Reply #666 on: November 24, 2016, 08:19:10 am »
One thing I was thinking about A/D noise of 2001, perhaps one can try using lower noise zener on ADC board. For example chinese 2DW232?  ;)

Hi my friend...

If you have one why not?  >:D Just replace the VR801 and ready to go... may be fine tuning on the bias current (R854 value) but yeah piece of cake.

What I don' t like in the K2001 design is that has a separated ADC VREF (not using the LM399 one) and moreover this one is setting also the bias current on the LM399 --> Why this is not the case in the K2000? And also in the K2002 (I could be wrong here.. just did 5 min investigation...)? ... so why only the K2001 is ADC noisy and there other two look good? mmmm

Just to confirm I have the right understanding (hope to be right):
VR801, 2DW232 are short time references: they don't jump but they drifts slowly in the long period ("worst" TC).
LM399 or  LTZ are long time references: they could have a pop corn noise (ppm jumping) but in average they are solid constant in the long period (better TC).

On the side, not ADC related, U328 AD707 (OPAMP for current bias on the LM399) why not replace it with something new? Didn`t check to much but it seems there are better OPAMPs out there pin to pin compatible. Are there other pin to pin compatible better devices replacement which could improve the references circuits?

Just writing straight off my brain.. sorry to all if I dropped something very stupid here, I have so much respect for all of you dragons thanks for reading this.
« Last Edit: November 24, 2016, 10:07:18 am by zucca »
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Offline TiN

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Re: Restoration glory of Keithley 2001 DMM
« Reply #667 on: November 24, 2016, 08:49:34 am »
K2002 ADC is very different to 2001's, and using lower voltage or *1.5x LTZ reference (from LT1043 flying cap blocks).
For 2001 you are pretty right, with autozero enabled 2001 measures LTZ ref, then signal, then zero reference, and based on these calculate the measurement reading. So stability of VR801 not need to be any special, as whole cycle is fast. There is also switching block to provide higher than 7V reference voltages.

Maybe Keithley was trying to get better long-term stability out of LM399 (to keep it still yet cost-effective 7.5d meter), and hence designed all this thing. On 2002 there was no such constrain, hence much more expensive LTZ1000 circuit and different ADC topology.
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Offline Zucca

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Re: Restoration glory of Keithley 2001 DMM
« Reply #668 on: November 24, 2016, 11:58:05 am »
So stability of VR801 not need to be any special, as whole cycle is fast.

Even more, according to here



the magic 2DW23x is even better than the holy LTZ1000, so my plan to put a LTZ1000 there is just blown in the wind...

Ok, who will be the first to test a 2DW23x there? If nobody shows up I raise my hand... it will take time since I do not have a 2DW23x available yet.
« Last Edit: November 24, 2016, 12:00:47 pm by zucca »
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Offline mimmus78

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Re: Restoration glory of Keithley 2001 DMM
« Reply #669 on: November 24, 2016, 12:17:42 pm »
I don't think it can be so easy to reduce noise ... but worth a try ... anyway I'm not brave enough to try it on my own meter.

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Offline Zucca

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Re: Restoration glory of Keithley 2001 DMM
« Reply #670 on: November 24, 2016, 12:24:13 pm »
I don't think it can be so easy to reduce noise ... but worth a try ...

yeah, there are a lot a variables to check (power supply voltage stability, noise from OPAMPs, divider, amplifier... etc.. ) but worst case we will know what is the impact of the ADC reference in the K2001 noise.

PS: FYI Kleinstein suggested here, a bigger C821>.33µF could also help.
« Last Edit: November 24, 2016, 12:35:22 pm by zucca »
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Offline TiN

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Re: Restoration glory of Keithley 2001 DMM
« Reply #671 on: November 24, 2016, 02:29:39 pm »
I'm brave! 4 time brave.
Actually thank to zucca and mimmus, now I narrowed issue with flaky meter to ADC board. Swapped board from 21 meter - noise is okay now :). Lets swap the zener, plonk 1uF polycarb cap on C821 and let her roll! 3458 beware, we are coming! Ha, not really.
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Offline mimmus78

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Re: Restoration glory of Keithley 2001 DMM
« Reply #672 on: November 24, 2016, 02:37:31 pm »
Well than you need to put another 200 EUR worth of components to use the LTZ1000 as reference and scrap this hopeless LM399 ... I may know a board you can use  :-)
This may be good for the 10V range, but unfortunately the rest of ranges remains in the same 6.5 digits league.
« Last Edit: November 24, 2016, 02:40:56 pm by mimmus78 »
 

Online Kleinstein

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Re: Restoration glory of Keithley 2001 DMM
« Reply #673 on: November 24, 2016, 06:16:25 pm »
Before exchanging the reference, I would you just measure the noise of the zener reference - at least of one has a suitable instrument (e.g. noise tester / AC coupled or low noise DMM (may need extra ref. to measure only difference)).

One point would be comparing the noise for reading a short and reading a low noise DC source.

Using the second reference and do the adjustment to the LM399 for every AZ cycle would be strange, as this would add quite some noise - though it would eliminate drift from the integrating resistors. Its a little like doing an simplified ACAL on the 3458 before each reading: it reduces drift, but also add time and thus noise, as less time is available for the actual measurement.

The more logical way would be a slower (e.g. average over 10s of readings) correction of drift from the reference and integrating resistors.

Increasing C821 would make sense if the zener is noisy - so the DW232 should not need it, the old zener might profit from something like 100 µF. The point is reducing noise in the 1-10 kHz range. The switching frequency for the ADC seems to be rather high - so jitter from the reference switching and maybe charge injection could be an issue. Also only switching the positive reference and adding a constant negative parts adds some noise. So the ADC is by design not very low noise.

The AD707 at the LM399 is not an issue: the noise of the LM399 is something like 10 times higher. I would be more concerned about switching noise from the LTC1043 might have an influence on the LM399.
 

Offline plesa

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Re: Restoration glory of Keithley 2001 DMM
« Reply #674 on: December 01, 2016, 09:30:32 pm »
One for K2400 SMU and second for K2001 :-+
 


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