i converted all to nV scale so there is no decimal points to consider on the Y axis, hmmm not enough samples to reach 1000 (i think needs about 2048 samples)
as kleinstein pointed out before, the K2015 is at the limits of its "anti noise" mod. now what would happen if the 2010 is modded?
I'm happy to do longer runs.
I've not looked in detail, but I'm more than willing to mod my 2010. (I need more DMMs, too much stuff to log)
1) change bypass caps next to important opamps. 100nF --> 10uF 50v. i used TDK x7r for this or something with very very high ripple character. 1210 sitting upright on 1206 pads. see pic
2) add additional DC reservoir to sections of the DMM, ie : ohms section has its own 100uF rails buffer, etc. i used some OSCON for this or any high ripple char.
3) gnd trace beefing, to bypass the 0.2 to 0.4ohm PCB trace resistance. essentially is = create new star grounding. the PCB ground seem to focus on a testpoint as center.
4) zero buffer have the most noise influence in my case, the entire section is overhauled. LTC1050 -> LTC2057, and the power float buffer all BJT is swapped from SOT23 to TO92. this section in my case heats up and creates alot of noise previously.
5) input impedance test, when bad, the impedance never exceed 11Gohm. when good, it is beyond 50G, this is when i have least noise.
6) front/rear switch, it is quite a huge problem with every touch creates a different offset as it is very old. so i did away with the switch totally. input is direct wired to the input protection section.
7) blocking of air movement / improving of air movement. this 1 is finicky. have pros and cons.
im not sure how to quantify if i did add 0.5digit of accuracy to this old K2015, i have nothing more accurate than this to measure. but noise wise i think it is seems no longer a 6.5digit. i think alex nikitin did the opamp mod too, he might have characterized his DMM with calibrated numbers?
and of cos the K2000 schema
https://xdevs.com/fix/kei2000/ this is from the original BBShot 38, there are some discrepencies, so always check before committing changes.
i posted some updates of schema here
https://www.eevblog.com/forum/repair/repaired-keithley-2000/, they are in kicad format iirc
option 8 ) if you did mod a bypass from front switch, then this could be interesting
https://www.eevblog.com/forum/projects/diy-low-thermal-emf-switchscanner-for-comparisons-of-voltage-and-resistor-stand/add a new auto/manual switcher?
option 9 ) PCB top mod see pic here
https://www.eevblog.com/forum/beginners/o-qn-on-funny-nplc-multislopenoise/msg808835/#msg808835 <-- these are the i'm bored mods, they dont seem to be very effective capacitor additions
**update @ dr dieseli remember now, after re-reading this post
https://www.eevblog.com/forum/beginners/o-qn-on-funny-nplc-multislopenoise/msg831474/#msg831474back then, i found some that NPLC setting creates a clock noise which varies with NPLC, can be "seen" in the final noise. and in that experiment, by using a base NPLC that is not 10, seem to create a less jittery noise. this seems to coincide with a later test which i used NPLC 8.