I attempted to repeat your test the best I could. I am using a few Caddock TK series resistors to make up roughly 1.9M. The two plots shown are my HP34401A (white) compared with the pre-production 121GW (red). I have also included the CSV file off the meter (renamed to .txt to allow uploading). The recording interval is 1 second for both meters.
Looking at the schematic, nothing pops out other than you should have a better reference. A bit odd you don't see the problem in every range.
Added your data set for completeness.
Hello Joe,
thanks for your tests.. the measurement of the 1.9M inside the 5450A by a 34465A, or a 3458A at NPLC 10 is as stable to the last digit, as yours with the 34401A, but w/o drift.
I could replicate that instability with other very stable 1M resistors and short, drilled cables also, but sometimes, I also get stable readings.. very strange.
So I wonder, what you mean by 'you should have a better reference', do you mean the 121GW internal reference resistors?
I doubt that these are the root cause for this instability problem.. because the Ohm mode is usually realized by a ratio measurement, so I suppose, that it's caused by the AD1.
And yes, this instability is visible part time in other ranges also, surprisingly in the 50 range, although that's driven by a high 470µA current.
Maybe you still have an old firmware running. In the latest 1-22, the counter register for the sampling frequency might be set improperly. Otherwise, 50 and 60Hz should be suppressed by 120dB @ 5Sa/sec, but 75dB only, when using a faster sampling rate... that could also be the case, as the sampling seems much too fast.
Maybe something inside the Ohm circuit is oscillating, or super sensitive, depending on the hardware version..
I did not fully understand the schematics, either, and also the HY3131 datasheet is really crap., still searching for an Application Note.
Anyhow, I really think, that this problem could be solved by software.
Frank