K2001 ADC

[Kleinstein]

I agree that the design with the 2 references is odd. The logical way would be to generate the current for the LM399 from the LM399 itself. Seams to be a little more effort. On the positive side the LM399 is not that sensitive to small changes in the current. It may mess up the calibration buit not much.

I consider averaging the zero as the biggest flaw with the K2001 (and many other Keithley meters) design. With a simple AZ cycle they could get way better noise (a little higher for 10 PLC but than going down as normal and not the noise plateau up to 50 seconds).
Another odd point is that the Ohms more seem to use some 5 V (may be up to 10 V) voltage range, that would be nice to have also for voltage, but AFAIK is not available.

The point for the the extra zener diode at the ADC is low noise. This should be rather low noise (not much worst than 2DW232), but has quite some drift, especially long term drift. The LM399 is a bit noisy, but low TC and low (at least specs for it) drift.
There is also resitor drift at the ADC and amplifier that can effect the overall gain. So the extra gain measurement in the AZ loop makes some sense.
There is still the flaw, that the 50 sec averaging time are not really enough to filter out the popcorn noise - though would need even longer filtering (like 10 minutes, if the ADC gain is not fluctuating too fast), maybe as running average to limit possible steps.

The size of the saw tooth part depends on both the zener TC and temperature drift.
If the sign of the TC changes the saw tooth should also change polarity.
For the 3.4 mA case the saw tooth still has the rising ramp that should correspond to zener voltage going down and if the temperature is going up, this would correspond to a negative TC for the Zener. The zeners have a negative TC with low current and may cross over to a positive TC at higher current. So the 3.4 mA may not yet be enough. The large saw tooth amplitude could be from more temperature drift, likely not from more TC.
It may still need more (like 5 mA).

[GigaJoe]

Hehe ... Kleinstein was right about 5mA  , according to pdf for 1N4579 2mA TC ==0 ;  good luck with that ...   even with 4 mA it drifting up but slower ... 
in my understanding zeners basically under the same manuf. process then it bin sorted ...  so for 1N4580-83  standard 4mA and maximum 8mA .. yeah

PS: if we look at national semi pdf, that was initially come from ..  they claim 6uV noise and 20uV maximum ... consider initial design was 87-ish , so it was a make sense to do 2 reference.  Bit 100% it was sorted 399 .. and that window of uncertain noise was overstated 

( i have really bad connector on ADC - take off back on - ADC overflow , clean, it back to normal , i shorted analog path across connectors, but seems even for digital it affecting timing or fronts i would say)

[iMo]

@GigaJoe: what is the tool you are using for the MADEV/ADEV??

[GigaJoe]

https://www.eevblog.com/forum/metrology/statistical-calculation/

[GigaJoe]

this is day and night with zener .... it actually CDI brand 1N4577 , same was used in Keithley 197 models ,  i would say soldering looks like original , but it literally thermometer ...

I really say it thermometer,  you need to imagine:   by design the whole board covered by a plastic antistatic cover , additionally i cover ADC buy few layers of a paper towels around, additionally i findout voltage on fan 8V (not 12 , someone add resistor i think)   so airflow quite minimal ....  And when i cover fan  - bam +50 microvolt up !!

i unsolder it and add some thermal insulation -  reaction to fan gone ...  it still have positive reaction ...

So a few questions again:
1 -  maximum safe current for zener , I assume around 10mA - limit , power dissipation etc ...
2  - assuming I want to replace it , how long it takes to settle to reasonable level ,  ( as well as consequence questions: ,  assuming i can select  2 or 3 with very close by parameters,   possible to do 2 or 3 in parallel, without any balancing - or this is a bad idea ? )
 
Update:  as usually Im' incorrect ...   

[Kleinstein]

Up to some 10 mA should be acceptable, but that would be a Zener that is poor (out of spec) to start with.
2 Zener diodes in parallel makes little sense - it would only work with additional balancing. For higher typical current (7.5 mA specs), there are 1N827 and similar, though some of those also get zero TC with less current. E.g. I have 2 with TC zero crossing at some 3 mA.

[GigaJoe]

Nah .. sorry,  data really was good .. Reason i unsolder, zener and did thermal insulation around.  But actually it TC level was  extremely bad 

so i hook voltmeter and decade resistor,
an actual current close to TC 0 appears in the opposite end,  around - 1.4mA
final result inside:  +25C  to +43C  , zener V  6.4XX35 - 6.4XX34 ; i got a negative but almost TC = 0

since Zener 0 it quite interesting to see calculation of autozero:
https://i.ibb.co/NdTcRW78/Clipboard-04-30-2026-02.webp


calculated noise RMS (from data sample of 7V connected):
original current 2.3mA , noise was -- 3.2uV
bumped to 4+mA  -- 2.8uV ( that where i got jumpy , as P-P was around 10-12uV )
now 1.3mA  -- 3.6 uV

for higher current , 1N827 , it no fact i got  better noise,  by spec voltage span 6.2 - 6.9, with unknown current for TC=0 , i need it alot to pick ..

[GigaJoe]

added 10uF 250V polypropylene  slug, to zener ADC, low leak , it over 50 Gohm , a bit help ....

I think it grand finale , here is result:
20V range , no filter , 7V source.
https://i.ibb.co/G4QVkpCd/voltage-deviation.webp
each sample around 2-4 microvolts jump now.

is anything at current source, to improve, for ohms ? ,or mA amplifier to opa189 ... i don't know ...

[Kleinstein]

With U322 replaced with an OPA189, the amps ranges already have an OPA189 as first amplifier. As this is a low impedance souce, the amps ranges where already relatively good (may be even slightly better, but no big difference) in the original configuration with a rather low noise amplifier.

For the ohms source I see litte easy improvements. One might gain a little by upgrading U324 (AD707), but the difference would be small and likely not worth the effort and messing with loop stability as newer amplifiers tend to be faster.

The main issue with all the precision measurements is the averaging for the zero and thus rather higher 1/f noise. So that would be a software thing to fix.

How good is the pimped version compared to the original ?

[GigaJoe]

ohh .. it day and night

for 20V as it would be the the weakest  initially P-P noise can reach up to 150 - 200 microvolts. even if you do 100 averaging it floated to 20 microvolts up and down and quite random for a long run. 

additional capacitors , especially for 8V and 6V ramps , additional for 15V , drop to approx. 40uV P-P and  around 15uV with heavy averaging .... and still floating up and down randomly ...

the breakthrough happens when i throw out NE5534 ... that was awesome and i understand that 40Y.O. chips likely not keep up ...

and second - TC=0 for ADC zener - that eliminated random wandering

so on rate of the most to least influencing:
NE5535 ( null detector)
TC=0 for zener
opamp that passed 20V ;  it was TL061,  later version BB OP124 - it was positive replacement for 20V range
8V and 6V ramps zeners capacitors shunt ,  as they derivates  from non stable bootstrap
15V additional caps
integrator OP602 , 2xLT1007 - they kinda OK so just 2 cents ...
ADC capacitors shunt 10uF to 680uF - minimal noise drop,  5V infuence a bit eliminated .. another 2 cents
10uF 250V  big polypropylene for ADC zener - another 2 cents ( so totally 6 cents - it matter )

on top:  bad oxidizes terminal switch , make noise - blindly shorted ; cold soldering on ADC connectors - randomly jumping like crazy. partially fixed, analog pins soldered some jumpers across connector.

with all this changes i think i leave it for a month running before adjustment. 

ah .. forgot a bit - some resistors elimination in ADC Vref circuit - im not sure even if helps .. it may be sensitive to ohm measurement , let say, where a minimal noise level coming in adc input.

a bit update, 
https://i.ibb.co/svDKRccX/yurhfgsfdhwtrgd.webp
still fan thingi not resolved ,   still wonder what is it .. spike are fan influence, it think it something to front end input,   as i keep shut fan longer it starting to correct, like offset.

[GigaJoe]

another problem:   ( call it off ;  unbelievable ... this an act of my reference .. many month ok , another battle )

startup, correction ... then bang ...

a saw such thing before, i thing,  at time when i bought it , as it was a while ... i think like this , occasionally

oscillation ?  ,  or a digital part ...

[Kleinstein]

I would not expect an issue from the digital part. Digital tends to be no so unstable and usually a big error and not just a little.

It could be some oscillation, it could be something getting to hot an going a thermal hot state. Another real possibility are contact problems or cold solder joints.

[bsw_m]

I had a similar problem with the built-in multimeter in my Agilent 34970A.
A ceramic capacitor turned out to be the culprit. I'm not familiar with Keithley circuitry, but it's possible a similar problem could occur here too.

[iMo]

Or a nearby amateur radio operator pushing 1kW into his antenna.. 
PS: the above reference is same as in the 34401A..
Btw., how did you identify that crappy capacitor??

[GigaJoe]

yeah it my external reference 7V ...  no .. idea , honestly  ;  hook ref. to 34411 2PLC sporadically jumping , take ref. out - works, put back ... good,  .. then jumping ... take off put back on  seems work ...  and as it suspected ....

noise as shorted:
https://i.ibb.co/9mVJvwTb/voltage-deviation.webp
that 2-4 microvolts ; and slow movement , guess autozero

[bsw_m]

Btw., how did you identify that crappy capacitor??

Analysis of the device's circuitry, assumptions about possible causes, several measurements for clarification.

[GigaJoe]

So I found my 2DW233 thingy... with the ADA4075-2, where one op-amp feeds the zener and the second is just a buffer.
It's slowly drifting down due to self-warm-up in a styrofoam box... (guess I haven't had much patience for the equilibrium point)
I switched to 10 AVE MOV, as it's usually the default — so 10 PLC and 10 MOV AVE; basically the default profile.
First wave is when the script starts — it always makes such a splash, then the device settles itself, slowly drifting down.
https://i.ibb.co/bjffc9xs/2dwsrc2.png

That's what the designers assumed would come out as the final product, I guess, in the late '80s — far from actual.

PS: We're going into season 2: modding the 2001 with 2 op-amps — chips arrive today!

[GigaJoe]

Season 2:   second unit,  made in 2005 ,

so far all this caps .. LT1007 , so far as the firs one
Adjusted TC for zener,   cold-hot +700 microvolts,  now -20


it was background discussion about null detector opamp with Kleinstein , he won,  predicting insignificance for replacement
(I'm do rigorous  now, check changes .. ;  how significantly was affection for the firs unit it only sense was ..)

( it BW images
Ne5534:
https://i.ibb.co/JRTDWM6q/5534.webp
TLV888:
https://i.ibb.co/hx39zRJx/888.webp
kinda 2 cents , if so .. a bit less spikes ...

my biggest disappointing are spikes: Jiggling with capacitors to find out noise source come with no result,  only integrator left ..
https://i.ibb.co/8npfdbwZ/spk.webp

[GigaJoe]

here is thing ...
the last image that "disappointing"  if average it , it would be sporadic wave up and down 5 microvolts around 10 microvolts between ...

this version has standard 2-opamp composite integrator with 10:1  1K:0.1K divider ....
Let try 1:1 , 100 Ohm removed:
https://i.ibb.co/Xr4gbn4W/no100.webp

100Ohm back, and 3.3K instead 1K:
https://i.ibb.co/yntnLr92/3K.webp

would be correct to say 3K better, probably try 5K  50:1 ,  and what i see without 100Ohm , basically integrator instability , or oscillation ?

[Kleinstein]

What divider ratio is needed between the 2 OP-amps in the compound integrator depends on the choice of OP-amps. The divider effectively reduces the speed (GBW) of the 1st op-amp. To avoid oscillation or long ringing the GBW ratio (with the divider) should be at least about 1:5. So a 1:4 resistor ratio for 2 equal speed OP-amps (e.g. OPA141). For less ringing one may want a bit more separation, like 1:10.
Details of the OP-amps and capacitance at the input can also effect the tendency for ringing.

What OP-amps are used ?

The wave up and down may come from averaging steps up and down in the raw data.
Are the measurements with a shorted input or with a 10 V reference ? Tests with external voltage are complicated by reference drift and possible 4 µV range size jumps of the LM399 reference.

[GigaJoe]

no it not an averaging ... 
 all plots 20V with shorted input ...

now it:
ADA4077   3K3 --  100Ohm - OPA1655 :   (1K - 100 , even worse )
https://i.ibb.co/S4NKZcpJ/4077-3-K3-100-1655.webp

testing with 50 Ohm now ... it seems better , need 2 hours to settle
preliminarily, if look at trend between 2AZ , 10uV spikes still exist ... but no 30uV ..
https://i.ibb.co/213h113R/4077-3-K3-50-1655-prel.webp

speaking of LM399 jumps,  I didn't observe such behavior on the previous unit at all , 5+ microvolt are clearly visible now, I'm
quite confidence, 2 reference design come up to mitigate this behavior as well,  for original NS LM399 they defined 20uV max as the noise, in 80x same as this design started, probably including that spice behavior as well ...  so i 'm pretty sure it averaged constant in the memory from many samples..

[Kleinstein]

With the ADA4077 and OPA1655 combination it should be enough sepration in the GBW to be stable even without divider, or only little divider. The rather fast OPA1655 may however need local decoupling, closer to the chip than the original PCB can provide. A little extra capacitor directly at the adater board may be a good ideal. One could check for ringing with a scope at the output of the ADA4077. If ringing is an issue, an RC series element (like 100 pF+220 ohms) to ground at the integrator input may help.

Quite some of the noise looks like on a long time scale, maybe the ~ 50 seconds lengh of the full AZ loop with averaging. This way offset drift (e.g. from instable temperature) could translate to noise. The 3 K - 50 ohms example looks a lot like a saw tooth expected from a constant offset drift in the background.

Reference steps would only be visible with a significant input voltage, not with the input shorted. The jumps from the reference are scaled proportional for the input voltage divided by the 7 V ref, voltage. So with 18 V in the steps get larger, ~ 10 µV.
The extra zener reference and averaging does digitally filter the LM399 reference noise, but this is not be enough for longer steps.
Attached I have one of my measurements of a 18 V Li battery. The DMM uses a LM399 reference and the steps are from the LM399 reference. The points are the average over 16 x 1 PLC conversions (24.5 SPS). The 3rd low phase in the example is around 30 seconds and would thus not be filtered fully with averaging over some 50 seconds. With a different LM399 I have seen much slower steps of multiple minutes, though that seems not typical. I am afraid that the K2001 even with improvements could be still too noisy to clearly see such effects, though the ref. noise could be part of the measurement noise.

[GigaJoe]

i probe by scope. nothing like hi freq oscillation..  but without divider it slowly jumping even harder ..... it really looks like high freq. oscillation in DC circuit . and second evidence are higher divider
let me try with 100 pf ...  "integrator input"  - are you mean between pins 2-3 of first opamp ?

yes that snapshot was early state , nonstationary , here after 7K second:
https://i.ibb.co/FqC8kTw1/4077-3-K3-50-1655-7-K-sec.webp

[Kleinstein]

The damping element would be between pins 2 and 3 of the ADA4077.

The dependence on the divider points to something like osciallation / ringing. One slight issue can be that probing with the scope can be enough to change things.

[GigaJoe]

wow ..  amazing , Thanks , it works

1uf for both opamp , 100 ohm removed , so 3K3 left , no divider as such ...
and , i forgot about that resistor that connect 2nd opamp output to -15V ...
that the only thing may add the noise.. i don't see wicked spikes ...
preliminary (always more exited as should be):
https://i.ibb.co/q3mfrpYD/4077-3-K3-NAN-1655.webp

?: back to season-1 , would be some benefit to change OPA140 to 1655 for a single opamp version,  or not so much ? better add-on board with 2-opamps .. it probably increase linearity in overall , a bit ...