The K2002 has some detailed diagnostics. One of the tests, 20.1 "Test Cal Zero" reports the direct output from the A/D converter for 0V; the 7V reference; and the 1.75V reference. These voltages use switches that are very close to the reference, and thus independent of the input conditioning, protection and scaling system.
The number is a 9-digit positive integer, and zero is represented by a number like 153,656,331. By running the test multiple times (clumsy, but practicable), you can pull enough datapoints to see what the A/D is actually doing.
In the table below, I took 10 readings. The last column is the standard deviation of the 9 3-digit readings; I only recorded the last three digits for reading 2 - 10.
The number represents about 1.2 counts per 100 nanovolts.
Therefore, the SD of the meter is about 400nV on the 20V range. This is roughly equivalent to 200nV on a 10V range.
The SD of the meter with a fixed voltage input can (rarely) get this low, or can go a bit higher. Depends on what the environment is up to.
The noise in the meter does not seem to see any incremental noise from the references.
At some point, I will need to implement TiN's op amp mod to the A/D board, and see if things change.
201.1 Reading 1 2 3 4 5 6 7 8 9 10 SD
Test Cal Zero 153656331 329 330 341 340 339 336 337 335 328 4.90 (means .5uV on the 20V range)
7V ref 240021466 452 454 463 448 461 450 457 458 458 5.02
1.75V ref 175247172 178 173 164 166 176 171 171 172 179 5.04