K2001 ADC

[GigaJoe]

so far things stall , i decided to touch 10nF
bought TDK 1kV , and some TW mfg. 1.2kV , 2kV , C0G of course, any basically over 1kV ..
measure leak:  as i expected - fail , 30V low noise V-source what i have, and 10M cur. sense - doesn't work , cap. exist or do not exist - same thingi .. , short by paper - numbers over the roof .. so it measure something ...

but what interesting thing - decided to clean up cap. from residue , took 99.99% isopropyl bottle special for electronics .. and numbers bump around x4 more ... then use my lovely contact cleaner - back to the same - nothing ..  non chlorinated brake cleaner ( super strong stuff) -  nothing ...  70% isopropyl  - wow bump up to 10 times .. medical grade isopropyl  -  a bit more  than nothing ..   acetone for nails - let say nothing ...  acetone from construction store -  x3 more that nothing ... and it has some correlation what it leaves on glass when dry up .  70% leave the most some residue.

[Kleinstein]

If the capacitor measures super low leakage, there is no need for even more cleaning.

To measure the leakage one may consider building a kind of integrator with the capacitor to test and a super low bias OP-amp (e.g. LMC662 or similar). Than charge and watch how well it holds the charge. It still need a good switch (e.g. mechnical relay / switch) to isolate the input path.
The same integrator would than also be used to look at the capacitor dielectric absorbtion: discharge (same relay/resistor) but change voltage to integrator output with extra switch in series).

C0G capacitors are not all the same. From may limited tests they are typically about as good as PP or PS and a few are better.

[iMo]

You have to measure the leakage in vacuum and at higher temperatures.
At normal atm pressure everything on Earth is covered by at least a monomolecular layer of water.
The contaminants may be dissolved in that layer..
 

[Kleinstein]

You have to measure the leakage in vacuum and at higher temperatures.
At normal atm pressure everything on Earth is covered by at least a monomolecular layer of water.
The contaminants may be dissolved in that layer..
 

If there is a water layer or not depends on the material and rel. humidity. A surface usually has a critical humdity at which a dense water layer forms that can caus extra leakage. Depending on the material this may be some 10% RH for a poor material or some 60% for a good isolation material.

The meter is to be used with normal air and can only rely on the self heating to keep the temperature some 20 K higher than environment and this way have roughly 1/4 the ambinet RH and thus operate near the dry end.

[GigaJoe]

flight to orbital booked ..

with failure of measure leakage ...  we moving forward to failure of DA ( Disaster Assur.. no-no  Dielectric Absorption .. )
here some curious paper i believe everyone read it already:
https://nepp.nasa.gov/files/25847/2013-Taverovsky-paper-NEPPweb-MLCCsVabs-n264.pdf

so 2 Lab PSU make 120V DC .. I charge cap, discharge for 10 sec according to paper , multimeter from short to cap - nothing ... I think military has some shielded bulletproof DMM that no picking any noise, internal charges, ets on >1G input ...  so fail again ..
Then i decided to use a scope - who care about resistance, i record a pulse ..

measured spike depend on discharge time - 10 sec discharging completely if something even appears lately it not detectable ...  so 120V charge - 2 sec- discharge ... 5 min to smoke whiskey ,and touch by scope ...
otherwise with 10S discharge need amp
 
1kV TDK - average 80mV
2kV - ave 60mV
... i dont have PP to compare  ( i mean MKP 4 10nF , 600-1000 V ) , if above are true it about to close to PP film: 0.01–0.02% DA ( pick from Internet .. )
and looks like such technique more less ok , but to compare apple and orange  need more less precise timing , maybe amp .. so automated ..

[Kleinstein]

C0G capacitor have very low DA and it challanging to measure. It need a meter / amplifier with very low input bias (e.g. low pA range, ideally less).
The rather slow standard test may not be practical with a small low DA capacitor. It is made for large capacitors. 

I did some tests on the DA of the capacitor in the integrator. This test is using a much shorter time scale, more like the ones relevant for the use in the MS ADC. So some 100 µs dicharge and looking at the reapearing voltage over some 100 ms or so. There is no need to test different discharge times: With low DA one can just look at the difference in the voltage from the effective discharge time.

The shorter time scale makes things easier with the small capacitors ( 2nF in my case).

[GigaJoe]

yeah need some automation ....

so i repeat test , REF - 2001M ;   DUT - one_opamp_ADC 2001  ;  don't need magnified glass to see a hump ..
source can't go over 13 ...  others are too noisy ,need to make one over 20
34411a as a reference would be better choice for this case .. next time ..

REF      DUT
00.000203   00.000200   0.000003                                                                  
01.145235   01.145230   0.000005                                                                  
02.024411   02.024364   0.000047                                                                  
03.085980   03.085850   0.000130                                                                  
04.020174   04.019970   0.000204                                                                  
05.097865   05.097712   0.000153                                                                  
06.135209   06.135155   0.000054                                                                  
07.051455   07.051428   0.000027                                                                  
08.134891   08.134891   0                                                                  
09.156290   09.156280   0.000010                                                                  
10.129100   10.129055   0.000045                                                                  
11.121470   11.121403   0.000067                                                                  
12.089108   12.089071   0.000037                                                                  
13.064085   13.064056   0.000029                                                                  

if it just integrator behavior , it probably solvable  .. if it something deeply .. well .. )

hook that 2kV 10nf cap ,instead what it was ..  , didnt made trends \stats ,  by naked eye- no changes ; P-P on short 20V range same stat around 15 microvolts ...

meantime  im thinking to replace OPA140 on integrator by something that i have .. and see changes .... the question is what it can be , just for test  ..   

[Kleinstein]

A 0.2 mV difference between 2 meters is a lot. It really should no be that bad.  I don't think just having a 1 OP-amp integrator would cause that much of an error. There could be different effects, like contact problems at the connector.
If one is lacking a good 20 V source, the 2 V range could also be worth testing. It uses the amplifier in a different way and the performance in the 2 V range should also be good, both for low noise and good INL.

[GigaJoe]

i think Tin did 2V and it was a differ ..
good idea to repeat it on 2V ,  do it later today ...


"like contact problems at the connector."
so switch back-front on that unit doesn't exist , basically soldered short ..  and i disassemble it yesterday to replace fan , clean all connectors and all around ....
Q810 indeed : 2N5432 stamped 5.68 on top .. ( selected one i think ... it can go bad ...  something voltage related changing condition ... i think 2V was a different shape )

yeah ...

[GigaJoe]

ok .. that thing corrected ....

use 3456a and K2001M , 75% 3456A + 25% 2001M = V-comb
this # also floated due to , not so stable source microvolts floating ,   still noise of 2001 , and i so lazy to automate, so old fashion pen\paper .. while mulling, a few microvolts off ...
(frac. of µV as V-comb not rounded as shown )
V_comb      DUT corrected   resid
10.004765 10.004767+1.7 µV
9.000491   9.000488   −2.8 µV
8.000520   8.000515   −5.4 µV   
7.000580   7.000574   −5.9 µV
6.000510   6.000509   −1.1 µV
5.000050   5.000047   −2.4 µV
4.000189   4.000187   −2.4 µV
3.000070   3.000073   +2.8 µV
2.000070   2.000072   +2.6 µV
1.000008   1.000013   +5.1 µV
0

[GigaJoe]

going futher ... season 2 episode with editors cuts ...

LM311 -  2 comparators replaced by LM211 ,  forgotten thing,  211 -has much better offset comparing to 311 ..
then i replaced null detector OPA140 to SA5534 ...  like lowest ... and it gettin worse ,   final TLV888 and x20 amplification instead x100. that give me some noise cut.  I still have around 20uV P-P for couples hours run,  but for a short window let say 200 measurement it 6 uV P-P instead as it was 10 let say....

As I see it: offset and propagation time are important for comparator ; noise, speed, and propagation time - important for amplifier. , so reason why at fast implementation , video amplifier and fast comparator or super fast for some 8.5D .

I just realize ... skale stretched x2 ;  it 20V but an actual voltage are half .. so actual measured noise twice amplified by calculation .... I'think i  reach a bottom of this...

last one: will replace LT1097 to OPA180 -  want less TC and bit lower noise ... 180 good enought .. (or 188)

[Kleinstein]

The offset of the comparator should not be that relevant. It would add a little offset to the ADC, but that is corrected anyway.
Similar the slope amplifier offset would not be that critical. It may make the rundown a little longer, but likely still fast enough.

Changing the LT1097 in the reference amplificaton to OPA180 can make some sense. I would prefer OPA207 or OPA205.
The OPA180 is likely good enough, but lower noise version could be used as well, as the circuit part is relatively low impedance.

[GigaJoe]

Here is integrator output , opamp pin6 - Cap. , this is single opamp version, current opamp now OPA189 :

some thingi not understandable for me, so let's go:
Set:  range 20V, 10PLC , input short.
(bit enchanted pictures .. )

Basically in sequence it measure something , like input , 7V ref,  short,  and here is sequence.
Bottom marks is when integrated cap. shorted
P00: https://i.ibb.co/b5pmxHrS/00.png
shorted long time, we measure something differ:
P02: https://i.ibb.co/fzg84Xrt/02.png
or short time, measure the same:
P04: https://i.ibb.co/fY5Q8X20/04.png
or twice:
P06: https://i.ibb.co/93ShkC8M/06.png
Hope someone may comment better ...

not clear why it measure like this:
P08: https://i.ibb.co/kLXwNm5/08.png
or like that:
P09: https://i.ibb.co/m5kK2zNS/09.png

P09 - probably scope picking noise , that 5 spikes  on horizontal line, but it correlated to steps ( why this steps ? ) on rising edge, and not exist on falling edge. - about 160kHz , and do not exist in certain time, so maybe it not a noise ..  bottom flat line P02

[Kleinstein]

The rather complex sequence in the 10 PLC AZ case is interesting. It look like there are 2 different zero readings. Also looks like 10 PLC for the actual input and 5 PLC for the Zeros and 7 V ref.

The pattern in P8 is relatively easy to explain. One can get zero from alternating positive and negative. This what happens most of the time and gives the higher frequency. In the center one has a pattern with 2 steps positive and 2 steps negative which also give zero, just with a lower frequecy. This can happen from the delay of reading the comparator to switching the reference. Depending on the start charge one can transition to that pattern when ater 1 step the charge is close to zero. I see the same with my ADC and I don't consider it bad.

The short ripples in P9 with the rising slope part are the extra switching steps to get a fixed number of switch operations. They are missing for the down part, as the down part is only one modulation step long. So switching is before and after the falling slope. With the opposite sign input voltage one would see the extra steps with the falling part, but not the riging part.

The sharp 500 mV spikes up and down are likely from probing or the ground. At least there should not be such spikes are the integrator. The switching part can cause some issues with digital ground current and this way it can make a difference where the probe ground is connected.
These short peaks are sometimes picked up in the first picture and sometimes not, as a kind of aliasing / undersampling.

[GigaJoe]

There is more ....

Ground point i use "1" , and according to P00, capacitor reset to "0" so waves never cross "0" it always above it .
( second CH scope, I didn't look at precise setting, so it basically for display not measure, as an output - TTL  level)

Output UB03 "ZCOMP"  signal:
P10: https://i.ibb.co/gMDztxC8/P10-UB103.png
Or I measure it wrong. but it triggering only when capacitor shorted and it basically zero voltage.

UB04, signal "CBCOMP":
crossing virtual "0"
P11: https://i.ibb.co/DD4R4YFq/UB104.png

 I don't really follow how it actually work. Major effort toward to to know when integrated capacitor shorted. and that , according to schematic are critical.
and "CBCOMP" - roughly let it go ...

[Kleinstein]

The k2001 has separate comparators for the run-up (CBcomp) and run-down (Zcomp). This is used to have to run-up always on the positive side and this way get the rundown always start from the positive side. This simplifies the run-down, but need a larger (like 2 x) integration capacitor, as half the integrator range is unused.

[GigaJoe]

opa182 finally arrive, i did small add-on board, 182 +1655, a decided to turn it standalone ,  input are func. gen. 50K resistor.
Due to gen offset integrator output tend to  move to rail , depend on offset polarity ...
Now: divider between opamp i intentionally set - NONE.
Even with that, the picture drastically differ comparing to simulator - no any spikes on triangle.  182 - generate spikes only when integrator output move to rail close enough, it starting limit output and 182 generate spikes of correction. otherwise - solid line ( i may not tune well was around 1V/div, as it always touch the rail when static, only change offset and it drifting to opp. dir. then you able to see solid line).

So major question about divider between:
Most likely the overall stability enforced by large 10nF integrating capacitor ....   Should i leave it -NONE , or for good sake add, let say: 2K : 0.5K adding stability ?  ( technically I can build non inv. composite with G=1, as a separate circuit and play with divider,   but my guess it a diff. topology, as the result) , or try very small integrating cap 50pF , and see the result ?

Upd.. screw it ..  replacement installed ...  I think i reach bottom line for everything ... about the same noise level for 20V.

btw ...  replace LT1097 in Vref. to OPA180 MSOP - small thingy , even try to find difference in PDF - no words...  so replaced , ON, measured OK , OFF , assembled. set to test - Turn ON - dead ... so one opamp (or both) shot during transition ON or OFF ... *&#$^$&$ !!! - replace it to SOIC8 OPA188 .. still alive...

at the current state all but one (OPA140 )  choppers , device became almost immune to temperature ,   closing fan produce a small spike of 20uV (vs 150 as it was) around an hour cold-hot to settle around '0' , 15min - close to zero ... Of course we have this and that ... but in overall ... it day and night ...

and as I'm deep in that rabbit hole ....  I thinking to replace  U324 AD707J ( Current gen. for Ohm) -> OPA188 ;  U328  AD707J ( LM399)  to probably on safe side OPA205A , or chopper - OPA188 ,  mostly for TC
and probably that the end ..

[Kleinstein]

The large integration capacitor is needed, as the FB modulation is slow and works only in the positive side during run-up.

Changing the AD707 in the ohms source would be tricky, as the same OP-amp is also used for very small currents and these would suffer from a chopper amplifier with high current noise.

The OP-amp at the lm399 ref. should not make very difference. It still ads to the reference noise. So the OPA205 should work, but is overkill.

[GigaJoe]

so yeah ...  it shows numbers ... then jump then back .. will work on divider .. maybe somethin else ...

ohm source -- damn right ... I forgot it really make low current .. it seems OPA205A ( ihave A!) to ohms and leave 399 as it ( but thing is ... it also reference for the current source .. and of cours they save a few cents using a lower grade )

Upd:
play a bit with divider ... stable around 1K:150 . but still some sense it jumpy .. the add 300pf to 150 resistor, and looks like that sense disappear ..

Then i decided to do this:  And i don't know is it right ? it over 1Mhz , does it affect something ?

[GigaJoe]

Episode 18 ... replacing 1 opamp integrator with a 2 opamp version ...

The divider above kinda doesn't make sense ... obviously. And whatever the simulator produces - hardly applies to ..
OPA182 - doesn't work well either ... speed and frequency don't do well for a stable result, after a few iterations I switch to OPA205A
And here the divider comes as a major player for achieving a stable result:
Original, as from main schematic, 1K:100
So after some iterations I got 5K:511 (I have 300, 400, 511, 670, 820, and some 1K 10 ppm SMD so some variations can be done)
It works more or less, making around close to 4+ uV RMS noise ... and moving from 5K up or down .. makes the result worse .. even without moving much ... like 5K2 or 4K8 - result became worse., 

So kinda a 10:1 magic ratio .. and I need to stick to it.
Like 6K:511 - total disaster ..
( start with 10PLC, 20V range , shorted input)
https://imgtree.co/direct/qWoSJ0Sv.png

Similarly, 4K:511
Then I move down: 4K, 3K, 2K with the same ratio .. where 4K, 3K give a bit worse result, 2K:200 - gives a very good result of 2.5 uV noise ..
1K:100 - therefore makes close to 4 uV noise ...

I have no clue of the reason for 10:1, even such result of going up or down by resistance ladder, but practically it is there ..
I have to involve Mechanical Mind (MM) for calculations, where it compares patterns, calculates noise per each 12-20 hour run, fragmentarily, like comparing specific time frame selections in a few runs and diagnosing which is better or worse ..
MM found out an interesting pattern that exists in measurements: 7 consecutive readings represent a wave up and down as average, and included it as part of the determination less - better, comparing 1, 3,5,10, 30 min fragments as less "wobbling" result etc...

So currently I do 500:50 which probably would be a bit better than 2K:200 .. ideally 2.1 uV , so P-P would be in 10 microvolt frame approximately , will see ..

it seems a second unit i need to review from that point of view , where no divider at all .... as well try ADA4077, instead opa205 , and maybe OPA187 ( regardless of elevated noise)  ..

[Kleinstein]

Normally the divider between the 2 OP-amps at a compound integrator should not be that critical. There can be a minimal divider for the circuit to start oscillation. With the rather fast OPA1655 and rather slow OPA205 or OPA182 it still has a good change to work with a direct link. A 1:1 divider should already be plenty. The exact limit depends on OP-amp details and integrator loading, so no exact general limits.
The divider is relevant for the AC compensation and stability, but absolutely needs no stable resitors. It needs changes by 10s or % to get any visible change - especially if not close to the stability edge. So any resistor grade should be good enough.

The 1:10 divider is already well on the slow side for the combination of OP-amps. For best speed it would be more in the range of no divider at all to a 1:1 resistor ratio.

The OPA205 is very similar to the ADA4077, kind of made as an 1:1 alternative. So the change should not make much difference.

Normally the divider ratio should not effect the noise, if at all they may be an effect on the INL of the integrator is ringing too much, or an OP-amp reaching it's slew rate limit. So the point to check is more using the scope at the integrator output or the output of the slower OP-amp. This is where on can judge on ringing and possible oscillation.
The choice of the slow OP-amp can effect the noise a little - so here OPA205, OPA182 or OPA145 would be slightly different, especially with the 1/f noise.

The curve shown shows quite some low frequency noise. Like some thermal oscillations and not using the AZ mode. I would not expect this part to depend on the ADC details. There may be something else wrong - a poor connector contact, supply hum or maybe the OPA1655 oscillating from insuficient decoupling.

[GigaJoe]

here is something 205 output ..
I tellin right away, it in edge of sense , board upside-down , and diff to reach, ground non ideal as well
time of exact measure - unknown , just random shots ,  probe x1 (for x10 - nothing ) , and a single, otherwise it just mess ..
https://limewire.com/d/AOS1i#Tq9qNQSRpq

[Vgkid]

*snip*

Limewire , that is a name I have not seen in about 18 years...

[Kleinstein]

The OPA205 output shows much of the expected signal. The square wave with some extra wiggles as expected for the run-up. There are however quite some extra spikes. This makes it messy. I don't think the extra spikes are all from poor probing as they are at odd / random times. So the meter may have an issue with a not so clean supply or the decoupling.

[GigaJoe]

So the meter may have an issue with a not so clean supply or the decoupling.

that is my fight for a while  additional caps everywhere , on top that ADC board itself has 6x680uF polymer, and 8-10 of on main board there and there.  adc itself as addon board , 1st opamp 22ohm +10uF and 2nd 5ohm +10Uf ,and 10uF across ramps for each opamp ..
So..  Sorry, miss shot.

it seems i understand now ...   lower resistor in divider decrease overall noise , as jumps between samples.  below around 150 ohm it a trend to decrease .. currently 50 ...   and upper resistor as ratio control let say authority of change  - too low (like 1:1 ratio)  i'm getting sudden spikes - as eventually noise , too low (12:1) "woobling" the result slowly floating up in down randomly in 5-10 microvolts interval with a few - to 10ish mins period..

so ... for example if i do no divider ... it works .. but generate random spikes , and quite noise. direct and 50ohm shunt ... a bit lower noise + spikes.
1K:50 (20:1) - despite quite low noise ,and now any spikes, random wandering
so i'm collecting stats for diff ratio as most optimal

If .. for example i like to filter ADC input , like in 34401 200pF + 100 ohm - an output gettin crazy and "0" up and down as there is no tomorrow .. so it doesn't work at all ..