I went through the whole thread again, but could not find any description, how your monitoring system works, and on which volt reference it is based.
Therefore, I could not draw any conclusion, which drift you really display here, i.e. really the drift of the DUT, or the drift of the monitoring system.
Would you mind explaining your practical setup, and how you are able to determine the absolute drift of the LM399s?
Thanks
Frank
Hello Frank, Ken,
the LM399 CH6 + CH7 charts are simple measurements of raw data (without offset compensation) out of my best 24 bit ADC (ADC#13).
Offset of this device is about 6uV which drifts around 1uV over the year. (negligible seasonal change).
ADC#13 is a well aged LTC2400 based device with a AD586LQ voltage reference and a temperature sensor.
T.C. is compensated by a 3rd order correction curve.
So this measurement is relative to ADC#13.
ADC#13 stability can be found by the following charts of my "daily" automated measurements.
These attached charts are done with offset compensation.
Offset is measured once at beginning of the measurement and subtracted from the following readings.
If I set day 0 at the beginning of LM399 CH6 + CH7 measurements you can see that during same time
ADC13 does not drift more than about +/- 1 ppm against two LTZ1000A references (blue and green)
which is also negligible against the 10ppm of the LM399 CH7 reference.
For the LTZ1000A references I try to get reliable calibrations.
But the only thing that I can say from comparisons to other instruments like Keithley 2000 with calibration history
or Fluke 5520A is that the drift of the LTZ devices is up to 2 ppm/year against those devices.
But I cannot tell wether the calibrator drifts or the LTZ1000A.
I will still need some years to make a final decision.
With best regards
Andreas