Could this be an effect of handling the devices? Are you regularly handling the devices or do you keep them in a "don't touch" corner?
Ok, I did not tell the complete story:
Usually my LTZ#1 + LTZ#2 references are running 24/7 on my measurement table and are only measured once per day with a wired relay multiplexer.
Once a week I also do a manual comparison against LTZ#3-LTZ#8.
1-2 times a year I try to get a calibration either from a friendly volt-nut or when the calibration guy at work calibrates stuff at our location with his Fluke 5520A.
The last successful automated measurement was on tuesday.
On wednesday + thursday the battery of LTZ#2 was down during measurement in the morning.
On friday I transported some of my references to calibration with the Fluke (unfortunately no measurement in the morning).
At work after 2 hours temperature settling I recognized that the battery voltage was unusual low on LTZ#2 (14.8V) although I had charged in the morning before transport.
At home doing the daily measurement after recharging I finally found out that against tuesday the voltage had increased by +22uV.
With the reading of the Fluke 5520A (with a 34401A in 100mV range as zero-voltmeter) I also get 22uV increased voltage difference between LTZ#1 and LTZ#2. So the +3.1 ppm shift was before calibration. (either the battery flat or the transport to calibration).
But on previous transports I never had any voltage shift on my powered LTZ references.
How does the change compare to a simple power down and thus thermal hysteresis ?
Shortening a LTZ1000 ref circuit is different:
With a simple power down I never have a voltage shift that I can measure (noise limit is around 2uV with 2:1 divider).
The setpoint temperature of around 52 deg C is rather low so that there is also low hysteresis between room temperature and powered LTZ.
After the shorting of the unbuffered output (see day 1250) it is of course different. In this case I could reduce the initial hysteresis by several power downs from a initial much higher value to around -5 ppm.
Hello Andreas,
when you overheated the LTZ1000 circuit at day 1250, you probably introduced a permanent hysteresis, which is of the type that does not slowly drift back by itself. The decrease of the output after overheating is the correct direction, anyhow.
Now, as the oven failed after years, the LTZ maybe simply jumped back in direction to the initial state, i.e. before day 1250.
That's also the correct direction, because if you cool down the LTZ1000, like below room temperature, or even below 0°C, its value will always go upwards.
I also don't think that it has anything to do with the back fed current.
Hello Frank,
I think that is the correct Idea. I usually had only short power interruptions of some minutes for tests.
Now with the battery flat the power outage might have been for 2 half days giving more time to relax into the previous direction.
Should switch off the device for a longer time to see what happens.
I also think that there is no correlation with any back fed current (since it is in same direction as normal current and much lower).
At the moment I make a longer measurement (~18 hours) to check if the Allan deviation behaves unusual.
with best regards
Andreas