K2001 ADC - Page 1
Forum section: electronics / general

author: GigaJoe
author_rank: Frequent Contributor
timestamp: September 17, 2025, 05:09:36 pm
content: looks like 2 version,approx 1993 , no composite711 should be where R811, just a single 602 as integrator .....like ... probably possible to add mod ada4077 + 2 resistors ....question: would it screw up something ( at most) , does it help with noise ( goal )

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: September 17, 2025, 05:20:49 pm
content: Can you provide some background on what this is, as well as clarify what you're wanting to do?

author: Kleinstein
author_rank: Super Contributor
timestamp: September 17, 2025, 08:34:18 pm
content: There are 2 different versions. One with and one without a 2nd OP-amp. The OPA177 has slightly better low frequency (e.g. 30 Hz to handle 1 PLC or 3 Hz for 10 PLC) noise than the OPA602. Still the integrator noise is only one noise part. One may not even see the difference.The divider at the compund integrator is used to slow down the precision OP-amp. So the relatively fast ADA4077 or similar OPA205 may want a different divider than the OPA177. The 1 K / 100 ohm in the schematics are a bit odd choice: rather low resistance and a bit much for the relatively slow OPA177 already. Beside a little less noise the 2nd OP-amp can give a slight DNL improvement: the constant impedance at the integrator is no longer important and less error from not having it. In theory there may be corrections in the software, so that a better integrator could result in a worse result from the corrections for an not ideal integrator.The ADC in the K2001 is quite noisy for different reasons, with no easy way out. In addition there is a gain of 0.5 for the 20 V range and the input buffer is also not super low noise.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: September 17, 2025, 11:33:15 pm
content: thanks,so " do not rock the boat" ....if i understand it correct: if i mimic schematic , 1.1K basically loaded 177 , skewed DNL, but as it constant , it may hardcoded in a software ,with part i dont have ...and if i replace 602 by let say composite , it still negligible due to overall design ...so in overall fight with noise the way by averaging only, means ... ideally to make this noise as close as possible to uniform , but ... electronics usually produse gaussan noise ..

author: Kleinstein
author_rank: Super Contributor
timestamp: September 18, 2025, 08:52:51 am
content: There are several reasons why the ADC is relatively noisy. One point is switching only one reference side and add the other side as on offset. This is an integral design point an nearly impossisbly to change. As a result there is quite some noise gain ( ~ 5.3) for the integrator (and similar also some other noise sources). The OPA602 is relatively low noise for an old JFET op-amp aready. This is not so much white noise, but the 1/f noise as much is about relatively low frequencies like 3 or 30 Hz for 10 or 1 PLC operation. With the somewhat complicated AZ cycle of the K2001 even lower frequencies may matter too.I have not looked at the noise in detail - maybe if I find time I could enter the numbers (needs still a few thoughts for design detail) to my spreadsheet.For the linearity the problem would be some hard coded (or after special factory calibration) corrections for the relatively slow OPA602. The simple 1 OP-integrator input acts like like some resistance ( 1/ (C_in*2Pi*GBW)) to the virtual ground. And this may need a correction for the fast run-down step, as it looks like this happens with a different impedance at the input. With the rather large integration capacitor they may still get away without a correction.For a compound integrator with ADA4077 + OPA602 I would more choose some 10 K and 1 K for the divider. One only needs an effective speed ratio of about 10 (can be as low as 6 with suitable RC to ground a fast amplifier that is good driving capacitive loads). No idea why they choose so much devider with the slow OPA177 in the other version.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 18, 2025, 05:43:54 pm
content: The point for possibly upgrading the integrator would not so much be noise, but the linearity. One could test for the DNL errors that are expected from the simple integrator without keeping the impedance constant. The expected error would be a saw tooth like contribution to the INL with jumps when the RU steps change. In between the differential gain would be a bit lower than ideal. One may be able to see the issue with 1 PLC conversions.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: September 19, 2025, 07:20:44 pm
content: it can be really difficult for me to test linearity even if improvedGuess that idea of change getttin discarded ...

author: Kleinstein
author_rank: Super Contributor
timestamp: September 19, 2025, 08:28:35 pm
content: The relatively high noise level makes INL testing a bit challanging. One could give it a simply try by looking a slow drifting voltage (e.g. RC discharge). If I found it right the Run-up modulation is 4.5 or 5 kHz. With 20 V range and 1 PLC this would make some 500 mV per RU-step.So a signal would be something like a slow RC discharge from some 1.5 to take some 15 or 30 minutes and record data at 1 PLC non AZ at the full rate.After subtracting a fitted exponential and maybe some averaging, one may be able to spot jumps that reat about after 500 mV. I my crude estimate is right I would expect something like 5 to 10 µV.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: September 21, 2025, 05:57:15 pm
content: second look at schematics :added polymer 100uF for zener 8V and 6Vinput short for max resolution no filter , 100 counts measured - 0.23 - 0.27 uV P-P ( 8V seems more important then 6 )10V with filter ave-100, has around 6uV P-P ; but this most likely contributed by V source .... pattern of moving back and forth aperiodic, guess some random spikes change the picture ...there is no circular moving of measured result as was before , it aperiodic now , most likely caused by source ....I'm kinda happy for such result ... maybe next round to change something ... who knows ... I need a better source and logging for now ..

author: GigaJoe
author_rank: Frequent Contributor
timestamp: September 26, 2025, 02:14:33 am
content: after all my mods: 20V range ; Ave - 100 frame9.9994009.9993999.9993969.9994009.9993969.9994009.9993939.9993919.9993969.9993949.9993979.999403 !9.999388 !9.9993969.9993939.999388 !9.9993959.9993999.9994029.9993979.9993959.999393Count = 22Min = 9.999388Max = 9.999403Mean (average) ≈ 9.9993957Median = 9.999396Range = 0.000015Standard deviation here ≈ 4.5e-06 (0.0000045)Median = 9.999396Mean (with all values) = 9.9993957-----------------------------------------------------------------------------( somehow it became jumpy that 388 , 403 excluded: )Count = 19Min = 9.999391Max = 9.999402Mean (average) ≈ 9.9993958Median = 9.999396Range = 0.000011Standard deviation ≈ 3.3e-06 (0.0000033)Median = 9.999396Mean = 9.9993958Noise of the source .. probably 2-3uV P-PI have 4 x 2DW233 and ada4075 , then averaging ... mostly made as low noise and reasonable stability for couples days ... ( 6 something volts output ) it about the same variation in numbers ...same behavior for shorted input , offset jumping on around the same value , so my guess ( i may wrong) it most contribution in floating and jumping behavior actually digital offset calculation ; couple hour give 25 rangeStationary oscillation around a baseline ~9.999395

author: Kleinstein
author_rank: Super Contributor
timestamp: September 26, 2025, 03:32:18 am
content: The digital offset compensation (AZ mode) should be relatively fast. It's purpose is to reduce the very low frequency noise. Ideally there should be not big jumps for the offset, but more thermal like fluctuations. My suspicion is that the offset correction does not work all that ideal in the K2001 (and also quite some other meters).In the Allan variance curves there is usually some extra noise bump visible already with a shorted input. The somewhat complex AZ cycle with addtional steps for gain correction complicates things and there can be additional jumps in the gain (some are seen with the DMM7510 during warm up). One point that could go wrong is the wrong internal zero point (e.g. at one end of scale, or anominal zero that is a bit off) for the ADC scale, so that gain correction could carry over to the offset.Tests with an external voltage add noise from the external reference, the meter internal references (ADC zener and long term LM399) and the gain corrections. The transition between the 2 separate references in the K2001 is a tricky part with not many things known. It looks like things are different for 1 PLC and 10 PLC (or longer). The longer than 10 PLC cases are likely just averaged from 10 PLC readings and not that interesting for the understanding.For the start and for the ADC noise the shorted input case is more suitable to look at ADC noise. I is the easier case to understand first.A good way to looks at the noise is the allan deviation with relatively long data sets, something like 10-50 x the longest time of interest. So this would be a few hours unter stable thermal conditions and well warmed up.p.s.Some of the visble jumps could well be from the LM399 ref. These commonly show popcorn noise jumps of the some 4 µV which would translate to some 6 µV with 10 V voltage.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: September 26, 2025, 02:10:11 pm
content: not sure about LM399 noise ... as the graph above NPLS -10 and averaging buffer 100 , basically 1000 measurement averagedhere 20V range , 10NPLS + 10 averaging frame , source 4x2DW233 ( 19mA about)and that it cycling ....same waves , and jumps ... so yes , P-P around 10uV for small windows and 25uV P-P for long one ( hours )