Am I the only one that doesn't see any problem?
niner_007 -> ΔY = 1.73uVpp (unkown interval)
Dr. Frank -> ΔY = 1.33uVpp (** 1 hour interval)
(** assuming 7V absolute value, and +/- 0.95ppm Δ)
So results doesn't seem too bad considering power cables, and board and DMM placement.
Hi folks,
That discussion is quite interesting now.
The LTZ is specified as 1.2µV
pp noise, which is mathematically equivalent to 420nV
rms.
The measurement from niner_007 shows 2µV
pp or 700nv
rms, if you look more carefully at the cursor position, which is definitely too high, and indicates additional noise.
Typical LTZs compared against a 3458A, or also against a 7510 (which has an LTFLU reference inside!?) only show 150..300nV
rms.
A 732A should measure even lower.
Please have a look into the 'DMM noise' measurement campaign, we did a few years ago. You will get a good estimate of the noise figures of the different DMMs.
Maybe the 7510 is more noisy for other reasons, (I don't like the limitation to NPLC 5), I do not remember that precisely any more:
https://www.eevblog.com/forum/metrology/dmm-adc-noise-comparison-testing-project/ These values depend on the settings, the equipment and the environment, and then also on the quality of the LTZ / DUT and the reference inside the DMM.
So to get comparable results:
1. Averaging time of the DMM: use NPLC 100, or 1.6 .. 2 sec averaging/sampling time for one data point
2. Always use statistics: Take 16 datapoints of NPLC100, note average and StD.
The StD is also equivalent to the rms noise value, on this time scale.
With more and longer averaging data, especially the StD, will be biased by timely or temperature drift effects, or from other noise sources like Popcorn noise.
3. You may distinguish the different noise or (in-) stability effects described above by using Allan Deviation statistics
4. Digitization error/noise: For sub-ppm measurements, at least 0.1ppm resolution @ 10V is required, but the 7510 seems to have 0.5µV only
5. Use a DMM with an equivalently noisy internal reference, like another LTZ1000, or an LTFLU. The real noise figure of the DUT is then about 1/SQRT(2) of the StD
6. Avoid or mitigate external disturbance, like E.M.C. and temperature change, which will both falsify / contribute to the StD value.
7. For precise determination of the absolute value of the DUT, use low e.m.f. cables, and reverse the leads at the DUT to cancel these.
On our MM2020 meeting, we discussed this data evaluation deeply.
Many measurements on the different LTZ1000 circuits (~22EA) showed 500nV
rms, or more StD. That was a clear indicator that something was wrong, and these measurements should have been discarded at once.
In the end we identified the root cause(s), so that we could reduce our disagreement of uncertainty between our reference groups from 2ppm to about 0.5ppm.
Scanning the DUT with different DMMs seems like a good idea, but the scanner itself will introduce a lot of e.m.f. errors / noise, or you invest $$$ in a nV scanner.
Notice again, that we're discussing sub-ppm
absolute values also.
The other way round, i.e. scanning several LTZs with one DMM, also depends on the quality of the scanner.
Frank