K2001 ADC

[GigaJoe]

there is schematic to replace 2 opamp version ..
https://imgtree.co/direct/2UPvJOk0.png  ( https://i.postimg.cc/sxbsptqm/2UPv-JOk0.png)
the final version 205 switched to opa187 , as 205 a bit .. like 4-5 faster as need .. considering nature of 187 so far i dont see residue ..

well.  i kinda hate Automatic Control System course in university , especially Feedback and Dynamic systems and Controls, and that fire back quite well in this case.
so divider between opamp became critical as for system in overall , not exact as separate for specific part ...
like example:
you have this behavior , more less ok ..   https://imgtree.co/direct/VfL1ehwK.png  (https://i.postimg.cc/v841VM6T/04-opa187-50-300.png)
and then simply change resistor in divider to 50 ohm more and getttin this:    https://imgtree.co/direct/39Vivr86.png ( https://i.postimg.cc/YSX9vyrq/04-opa187-100-300.png)
or this:  https://imgtree.co/direct/IkDrWaat.png  ( https://i.postimg.cc/63yQnLTt/04-opa187-100-1000.png )
I have no explanation , as it appears random islands of good bad mediocre divider ratio

so i simply log variation of hella lot of samples, an throw all in AI to find the best , with condition lowest momentum noise but priority of minimal wobbling in 1 3 5 10 15 min range. So noise bad but if system in overall wandering up and down - it even worse....

and speaking of noise ..  it appears that noise injected in adc ( surprise, surprise   ) and  that appears as pattern  in output voltage in most 0.28Hz with 2nd 3rd harmonics 0.56 0.84 .. and that noise added , generate 7 samples pattern  that about 20% of total ...  So best averaging for example not 10 .. 20  but X7
A second thing .. decreasing divder ratio decrease noise , but increase wobbling, it starting to flow in a few minutes pattern ...

adding shield to ADC , partially to analog part  cut another 30% and some nasty spikes ... so the result:
20V range, input short, 10NPLC, no filter, 4 hour warm-up
https://imgtree.co/direct/COfOS5Ll.png  ( https://i.postimg.cc/nV9zLk6M/COf-OS5Ll.png )
I dont think  I can squeeze more , maybe a bit for another few month ..

[HairyWombat]

Q810 is supposed to be 2N5432 as a rather low R_on switch. Like with most JFETs there can be some variations.
With 12 V input one would get 200 µA or 1 mV drop for 5 ohms R_on. 1 mV drop could roughly change R_on by about 100 ppm or 50 mohm. This would be roughly a 2 ppm change to the 57 K parallel with 40 K (from the zero shift). So one could expect up to some 2 ppm effect / INL error at 12 V.  It would get better if they have some selected for low R_on / high threshold (both helps to lower the nonlinear effect).

I am currently repairing a 2001 and tracked the fault to the ADC, fault symptom is complete lockup when accessing the ADC on startup.
I wont go into detail here and will give a full photoshoot in the main 2001 repair thread.
5V was slightly down at the ADC and the OPTOs seemed ok but the data is just slow bursts and does not look right.
Swapping the ADC from another working 2001 I have restored operation and she passed all tests, beauty.

Now I found Q810 with its gate and drain never soldered when I first started on this meter and thought that was strange for a flow soldered board.
No evidence anyone has been in here at all so just soldered them and moved onto caps, cleaning etc.
Once i new it was the ADC I fired up the thermal again which led me straight to Q810, ASIC was warmer than the good board also.
Q810 Drain to source measures about 4.5 ohm hmmm well thats stuffed.

Any recommendations on a replacement for the obsolete Q810 PN5432 N Channel Switch?
I have MMBFJ108 SMD and J109 TO-92's on order however, as is always the case, they are not as good as the original RDSon wise.
Plan to shoehorn a 2N4393 in there temporarily from an old HP meter board to see if it fires up fixes it.

[Kleinstein]

The MMBFJ108 would be a good replacement. Fairchild AN-6609 given process 58 for both the PN5432 and J108 (the THT version, that may be hard to get).  R_on would still scatter a little with the threshold.

For a start the 4393 (or slightly better 4391 / 4392) should also make the ADC work, just with some extra TC (e.g. 5 ppm/K range) and maybe INL (could be a few ppm). So the exact replacement would only matter for performance, not for having the ADC run at all.

4.5 ohms drain to source could be normal. That is in the valid R_on range.

[HairyWombat]

The MMBFJ108 would be a good replacement. Fairchild AN-6609 given process 58 for both the PN5432 and J108 (the THT version, that may be hard to get).  R_on would still scatter a little with the threshold.

For a start the 4393 (or slightly better 4391 / 4392) should also make the ADC work, just with some extra TC (e.g. 5 ppm/K range) and maybe INL (could be a few ppm). So the exact replacement would only matter for performance, not for having the ADC run at all.

4.5 ohms drain to source could be normal. That is in the valid R_on range.

Thanks Kleinstein.
The 4393 didn't help and suspect there is more mischief afoot, some of the in circuit measurements on the other JFETS don't look good.
Good point that it could be valid as the gate was floating when I measured DS but I doubt it, was a constant 4.5R both ways across it all the time.
I havent done much with JFETS, never had the need so excuse my ignorance.
Hooked up a test circuit and am unable to get Q810 to turn off completely, measured 0.7V G to DS
I pulled Q809 and it performed well in the test circuit, measured 0.6V G to DS.

I can see the CPLD pulsing part of the circuit every second or so like it is trying to settle it maybe?
The fact the CPLD is warm is not good when the other one that is good does not get warm at all makes me think it may be shot.

This is the good ADC with the cool as CPLD in the upper left.


This is the dodgy ADC, CPLD visibly warm on the left
U802 the NE5534 and Q801 run quite warm on both ADC boards equally.

The 5V being pulled down a bit also doesn't bode well for it.
What could have done this but not effect anything else on the analog board, dodgy rails from bad caps maybe?

Have a feeling it wont end well.

[HairyWombat]

Additional photos of the ADC's I should have added

[Kleinstein]

It is normal that the ne5534 gets warm - they just have a rather high supply current.
The ASCI (CPLD) temperatures could be different from different versions (newer may use less power).
The transistor that looks a bit warms is Q801, which is a driver for the reference. It could be normal to get a bit warm with some 3.3+3+0.5 mA and some 5 V.

To see how much of the ADC is working one could look at the integrator output.

[HairyWombat]

I figured the NE5534 temp is normal, I have worked with them before.
The thermal cam and second board allow for very quick comparisons.

As to the processor I also theorized that an older one might not be as efficient.
The faulty board has an Altera EPM5128LC with a sticker on it for the version while the newer working board has a custom print on its CPLD.

I am new to complex ADC's and Integrators so I will keep reading, learning and poking around.
Again having the working unit makes for easy comparisons.

Update: Swapping the CPLD between boards revealed it is fine.
It just draws quite a bit more current, so I am now certain it is one of the op amps or FETS that is shot.
Meter started up without the CPLD fitted to the dodgy ADC so guessing values are miles out as such it keeps waiting for stable levels.

[GigaJoe]

@Wombat:
what on the device screen ?  "buffer overflow"  or "----------- "   like no data ?
you have a ADC , old type - 1 opamp,   2nd newer - 2 opamps , witch one toast ?
" could look at the integrator output."  - opamp otput pin,  should be triangular wave approx +3v from ground as median.

@Kleinstein:
back to my topic:
I'm kinda not certain .....
During some testing, I lost a 10 µF cap across ±15 V and got wild system behavior. I'm questioning whether the second opamp (OPA1655) is on the edge—is it reason to keep it and add ceramic caps around to avoid oscillation, or replace it with something less fast?

I hooked an OPA172 as A2 for a test. The combination OPA187 + OPA172 works, no divider ,no 10uf cap across, but the first one (OPA187) isn't fast enough (noted ..) . At the same time, for amp-1 , options like OPA205 and OPA182 are stable to some degree (still jumpy) under very aggressive dividers like 15:1. 680:50 ,  ( i think slew rate a critical )

So I'm wondering: should I have OPA1655 as A2, as the 34401 kind of does well with AD711, which looks bleak compared to 1655?
something that faster than OPA187 ... and something that slower OPA1655 ...   ideally no divider ( it visible increase noise , i guess by amplifying on divider , not like  Johnson noise )
ADA4077 + OPA140 - ?
OPA196 + OPA140 - ?  ( i may order 191 , if 196 work well ... )
196 + 192 ?  ( pretty much need some divider )

PS... 3rd opamp as follower at output to isolate integrator ?

[HairyWombat]

@Wombat:
what on the device screen ?  "buffer overflow"  or "----------- "   like no data ?
you have a ADC , old type - 1 opamp,   2nd newer - 2 opamps , witch one toast ?
" could look at the integrator output."  - opamp otput pin,  should be triangular wave approx +3v from ground as median.

Thanks GigaJoe it just stays on "----------" If i press Auto the auto indicator turns off and then nothing further.
If a random button is held on power up you get a different random upper key function and nothing else.

Funny I didnt even notice as it went back in after the photos, it is the single op amp version that is sick.
Yes I probed the OPA602 output and it sits at about 8.6V with a bit of a pulse coming out.
Its input was just a minor ripple so something is up here.
Pulled C825 and get a constant 16-17mV over a 10M resistor across the voltage range so seems leaky.
With it out I can see a good positive to negative transition here.
I do not have anything good enough on hand for C825 so placed an order including a lot of the OP Amps you guys mentioned.
Want to avoid doing anything to the working ADC so not pulling parts from it.

I have worked my way across the circuit from the left and as far as I can tell all the references and CB logic switching are ok.
The LM339 reference is good and the lower first 2 (a&b) check out however I am sus on it so will replace it.
There is no activity from CBCOMP or ZCOMP
Not much happening on the NE5534 output
The center fet switching circuit has done my head in a bit and do not quite understand it but still sus on the other fets I haven't tested.
I do not want to just replace heaps of stuff as some has been specifically selected.

Is there a circuit for the single op amp version by chance?

[GigaJoe]

yeah "--------" no integrator activity but logic should be alive ...
basically to test integrator , you may replace it by any, let say TL081,  hope you have one.

then you will see triangle, but device will start random numbers noise in the same time  ....
so you can try , selftest , it may pass or throw some error's due to TL081 ..

C825 - you may replace by any ceramic for test , as the target to make it running, later on get a good one.
circuit for single same as dual except op177 in integrator  -that all

[Kleinstein]

For the broken ADC, a rather fixed 8.6 V at the integrator output would cause no action at the comparator.
In this case the FB signal (R835/R836) should be most of the time positive with some GND pulses.



For the 2 OP-amp integrator there are many options that should work.
A2 should be fast, ideally low open loop Z-out, but no need for precision. In my ADCs I have tested / used so far:  TLE2071,OPA172,OPA1641 and they all worked similar. Other candidates would be OPA197 (rel. low power, but this may come with weak output), OPA1677  (SOT23-5 not ideal), OPA1655 (high BW, but high power consumption). The offset does not matter (A1 can compensate > 100 mV) and thus no need for an OPA140 or OPA192. The AD711 in the 34401 looks indeed a bit slow.

A1 should be low drift / low LF noise and ideally not very slow (e.g. >= 1 MHz GBW). The OPA177 in the original K2001 2 OP-amp version is a bit slow, but seems to work good enough. The main candidates I see are OPA140 (or related OPA1641, OPA145), OPA205 (or similar ADA4077), OPA182, OPA387 (with +-2 V supply).

I am still confused that the divider has so much effect on noise - it should not. Much of extra noise looks jumpy, not like normal noise. The divider would effect the stability / ringing of the 2 OP-amp integrator. Strong ringing could cause INL/DNL errors and maybe a little extra noise.

An an extra follower would add delay and complicate things. I would more consider replacing R826 with a current mirror.

[GigaJoe]

R826 - gone, i think i specific to original chip . If i introduce it , device make large waves to few millivolts on shorted input ...

so ... no cap across A2 - bad behave,   only specific range of divider otherwise - bad behave,  work with  very slow A1 or bad behave ...
everything lead to edge stable A2 and, i thing, a reason why i have islands of bad\good divider, and sensitivity to A1 ....
but .. ADA4077 + OPA1655 work on second one with more less OK, caps across rails, it oscillate if no caps- so stability again close to borderline.  yeah ..

i can not decrease integrating cap, as it hardcoded in the code ... 10nF  comparing to 400pF or 1nF  it quite a bit different req. to A2 ... I think current output are matter ...

btw,  OPA1677  appears in SOIC8 ,  and new OPA1688 - design for low impedance load  ....

speaking of noise ... it not a thermal noise , i think it combination of pick everything inside + digital mgm. on adc board , something else ... and no divider increase wandering ... but i cant tell now as it , I think, side effect of stability ..

here is 187 + 172 , no divider, output over few hours:
https://i.ibb.co/kgkzBtDc/17-OPA187-OPA172-300-000-P2.png
1000 samples:
https://i.ibb.co/xSY5gFcd/17-OPA187-OPA172-300-000-P2-1000.png
( it actually not so bad .. )
... maybe 200pF -100ohm would work now on input , before it throws the chaos

[HairyWombat]

.
basically to test integrator , you may replace it by any, let say TL081,  hope you have one.

then you will see triangle, but device will start random numbers noise in the same time  ....
so you can try , selftest , it may pass or throw some error's due to TL081 ..

C825 - you may replace by any ceramic for test

Nope I don't have a TL081 but I do have a TL071 that I have had since 1985, I know it had been breadboarded when I was younger.
This is why you keep everything as you may need it one day but 40 years? Can't believe it still works. Found CA3030's too wow.
Soldered it just in the holes for easier removal and left the MKS 10nF with worse leakage than the original in there.

That was it and 2001 No.1 is alive
The OPA602 was shot but why, died of old age?

Thankyou guys.
It is quite ironic that you are discussing the exact section of circuit I have issues with at the exact time I am investigating it.

I have ordered OPA827AIDGKR and OPA1655DR (added to orders quickly while reading you guys) which do you recommend I use long term?
For C825 I plan to use FKP3F021003F00KI00 but also got a few metalized polys in Kemet Wima and TDK.
Surprised it worked with the double leakage MKS I have bodged in there and the original would no doubt work too.

Just the one error 200.3 A/D Fast TS Circuits Incorrect, to be expected
Probably that MKS cap wouldn't help and I still have that dodgy 4393 FET in there for Q810 also
Last 2 digits bouncing around badly but it is measuring.

Now have a lot of parts coming I do not need

Entire repair blog over here https://www.eevblog.com/forum/testgear/restoration-glory-of-keithley-2001-dmm/900/

[Kleinstein]

The fast OP-amp need good decoupling / like the capacitor directly at the OP-amp supply. The OPA1655 wants them with short leads - the ones on the ADC PCB may already be a bit far away. A slower OP, like the OPA172 is more forgiving, but still
That is normal and not a bad sign for the integrator.

The OPA602 could have died from heat cycles or ESD.
For a replacement in the 1 OP-amp version, the OPA827 is much better than the OPA1655. The low frequency noise does matter and here the OPA1655 is not great. A OPA140 / 1641 would be another alternative (still better than the original OPA602).

Capacitor leakage would effect the linearity. Similar the dielectric absorbtion of a MKS capacitor would cause linearity erros, that can well be visible.
The suitable capacitor types are PP, PS and good C0G MLCCs. In my tests (2.2 nF) they were roughly similar - with a TDK MLCC quite a bit better.
The fast ADC error may be one of the JFETs for the fine slope - AFAIK the fast conversions use a different fine slope.

[GigaJoe]

last 2 digits  random -  that a normal behavior of 2001 , reason why i didn't use it,   till starting to mod it...

200.3 A/D Fast TS - it fast integration circuit fet with 800k, generate current instead 4M, it running only on the fastest settings, 10ms integration, and not on 11ms,  - absolute useless thingi ... you can try fastest mode, i'm pretty sure it will work , that 200.3 - likely false positive.

currently i'm not sure if OPA1655 are ideal for dual opamp as the second one ... and 100% not for a single.  For a single opamp integrator i had OPA140 works,  but cant confirm 100% as linearity was toasted due to my mistake ... i did just in place replacement , and no R826  , you can play to bias it, but my result was awful , agree on OPA827 , and again .. R826 probably need removed or adjusted.   

for integration cap i use something like this:
https://www.digikey.com/en/products/detail/kemet/CKC21C103JEGACAUTO/11696189
look for  10NF 1000 - 2000V C0G 2220
or you can throw money to:
https://www.digikey.com/en/products/detail/knowles-syfer/404021K00103JQRAF9LM/16615124

[GigaJoe]

@Kleinstein
"The low frequency noise does matter and here "
I'm not arguing about it,  the thing is ... see on my chart ... the best what i have ,  ignoring wandering was around  10-12μV P-P or  around 3μV RMS, in that situation it 4 nV/√Hz , or 8 seems doesn't matter. Like in theory yes,  in practice - see trends ..  But frankly ... OPA205A  make the best level of noise,  and heavy low wandering ever.  need a compromise between this this and that ... for this specific device ...

 

[Kleinstein]

There is no need to use extra high voltage capacitors. More moderate 40 V / 100 V types should be good enough. The PP types however naturally often come as some 400 V or higher.

The OPA205 is good in the 2 OP-amp integrator(with a suitable fast OP-amp), but not as a single OP-amp.

There is also still noise from the amplifier (especially with original LT1007 in the 20 V range) and the JFETs from the input buffer.
Overall the K2001 is not low noise. Modification can improve somewhat on the noise, but there remains the poorly working AZ mode, that fails to suppress 1/f noise above some 0.01 Hz.

[GigaJoe]

i think 1kV or higher a safe bet , due to low frequency of integration, high capacitor, and primitive implementation ... like better safe than sorry ... I use 2kV personally ..

[HairyWombat]

Error 200.3 was caused by that 4393 FET, is passing all tests now
It obviously wasn't switching enough and the original Q810 is ok.
Tacked the original Q810 and C825 back in and plugged it back in without extensions.

It has settled down quite a bit but not as good as 2001 No.2

[GigaJoe]

@Wombat
do you have an option to hook something and get a plot ?

[HairyWombat]

Depends what you mean, what am I monitoring and over what period. TC?
I have plenty of meters but if we are talking 6.5 digit I havent gotten right into the pants of my 34401A yet but have the lead.
There is a full list of my current stash of gear here
https://www.eevblog.com/forum/buysellwanted/wttfs-(au)-datron-wavetek-4600-autocal-transconductance-amplifier/

I have a GPIB to USB lead but have done nothing with GPIB as yet.
If you can link to how and which software I would be happy to contribute

[GigaJoe]

to get data over gpib and make the trend what actually it measuring

[HairyWombat]

Sorry I hijacked your thread guys.

After running shorted tests with the shield back on I have concluded that there is 4 - 5 times more noise than 2001 No.2 although it's not great either.
Can the standard noise level be improved as it seems counter productive for a 7.5 digit meter.

@GigaJoe: Let me know what to do and I will do what I can, this interests me.

As to the cap choice yeah nah to a $20 SMD cap, nice one Joe maybe for that fancy custom one you are building.
From what I gleaned a polypropylene only cap is preferred for an integrator however a metalized version will do but not as well.
Full through hole polys are harder to get but the FK3P fits that requirement even if only 250V.
Whilst a higher voltage cap will generally have lower leakage at some point you hit the wall of diminishing returns.
But I am most likely talking out of my...

What are the chances of designing an adapter board that drops into U809 and modifies a single version into a dual op amp integrator easily.
Or would this have no chance due to parasitic interference etc.

OK it sounds like it is not worth considering as the gains are not worth the issues it could create.
When you say low level calibration is that beyond the ability of most calibration labs?

[Kleinstein]

An adapter to U809 for a 2 OP-amp solution should be possible. Parasitic capacitance should not be an issue. The integrator input already has quite some parasitic capacitance and other meters even add some extra capacitance to ground there. Ideally one would want a seprate ground connection for decoupling of the extra OP-amp.

The good point of the 2001 is more the 20 V range and a relatively low TC, not low noise.
There are a few points to improve on the noise - so not all from one poor part, but several weak points: For the 20 V range this includes U341 (AD548), U322 (LT1007) and U809 (OPA602) in the 1 OP-amp integrator version (the 2 OP-amp version should already be a bit lower noise, even with the OPA177). The 2 V range would be less / hardly effected by U341 and U322 - so the 2 V range may be performing better and would be worth extra tests.
Another point could be adjusting the zener current on the ADC module to have lower TC there, for less reference noise.

A big contribution to poor noise in AZ mode is the way how the firmware caclulates the result in AZ mode. It is not suppressing 1/f noise well. In the non AZ mode the performance is not that bad compared to other meters. A fix would need an improved / fixed firmware, which I doubt we will see from Keithley for the old meter.

[GigaJoe]

changing opamp it a fraction of work,  suppress rails, and especially as they using non-filtered bootstrap to feed differential fet input, in assumption it has infinite PSRR - it hilarious.  zener TC - another  "feature"
 but in the end still ... maximum u get it like this:
https://i.ibb.co/8gMdNx3x/voltage-actual-centered.png  20V shorted ,no filter
with 14 average,  it will fluctuate , ±5µV - about ...  im trying to squeeze it to ±3µV - and call it a day

probably draw on schematic would be most useful

but you need to read data and collect plots as only the way to see behavior , second you need very stable reference to measure for a few days and see if device not drifted or jumping or simply oscillating and shows Mars weather ..

then keep in mind : total redo need a low level calibration, 2001 has regular artifact calibration,  need precise 2 voltage , and 2 resistance to calibrate everything ,   changing internal thing, need low level calibration as well.
lastly - there is chance you may trash INL , as i accidently did , without precise measurement ,   hard to detect.