Hello branadic,
LTZ#2 was intended to show the drift of the ADC reference.
LTZ#2 itself has most probably a drift of around -2 ppm / year.
But since the calibration standards that I use to calibrate LTZ#2
also drift it will need some years until I know the whole truth.
By the way ADC13 drifts around the same amount as LTZ#1.
LTZ#1 calibration values show a lower drift than LTZ#2.
(so I should have used this as reference, but who knows in advance).
The common mode effects are partly due to environment
humidity + temperature. Temperature is compensated within ADC.
But nothing is perfect. And over the time also the contacts of the
measurement setup get loose and have to be moved/tightened again.
Attached the ageing curves of ADC13 compared to my
"stable" standards over the same time window.
The conclusions from my side are:
you will need minimum 200 days (half a year) run in time
until you can start qualifying the LM399.
In this case I would prefer LM399 CH7 which shows the higher stability.
(less standard deviation)
With best regards
Andreas