More information about my measurements:
1/ NPLC 100
2/ Readings taken every 24 hours
3/ DMM warmed up at least 4 hours before each reading
4/ DMM turned off each day
5/ Autocal performed immediately before each session
6/ Lab is temperature and humidity controlled (plus/minus 1 degree C)
Thanks Soulman and Dr. Frank!
My conclusion is that the DMM internal reference is moving a bit ... perhaps due to the on/off cycling. The external LTZs are powered constantly. Autocal shows little to no change. That would indicate that the A/D board is solid?
TiN, how is "superstable" LTZ1000 reference project coming along?
Yes, measuring once a day, but shutting off in between is not the best method.
Anyhow if you 're using the 3458A intermittently, like on a monthly basis, like here:
https://www.eevblog.com/forum/metrology/hp-3458a-ad-board-spare-part-availibilty-and-cost/msg3156428/#msg3156428Then these fluctuations should be 1/10 of yours only, i.e. +/- 0.2ppm from day to day. Especially this common dip +2ppm at day 6 makes me wonder, what's going on.
You also need to take each measurement with sufficient statistics, i.e. with standard deviation, so you can see if something is fishy.
Personally, I take 16 readings @ NPLC 100, with statistics, for each reference and for a single measurement point.
If the StD of 3458A vs. LTZ1000 is from 100nV to 300nV, then the measurement is ok.
You should also check if your used cable is ok, i.e. low thermal gold plated or pure copper. This can be checked by reversing the cables at the DUT and comparing if + and - measurement are identical to about or below 1µV.
I would also recommend, to do a 24h stability monitoring , 3458A vs one of your LTZ1000, and with TEMP? monitoring, whether you get such stable curves like mine.
From that you can very often deduce, what's wrong.
Frank