Hi,
Did you notice this happening repeatedly or with different references? I'm just wondering if this is a fundamental issue, or one which can be fixed or reduced by things like component selection, circuit design, processing and usage.
This hysteresis is a fundamental feature of the LTZ1000 reference chip itself.
Description can be found in the Pickering patent, and also in different papers about his Datron / Fluke 7000 reference.
Another hint is inside hp SN-18, where they describe relaxation effects of their LTZ1000A references, which is a secondary consequence of hysteresis.
I observed that effect on 4 of my LTZ1000 references experimentally, 2 of them where sent in winter, when outside temperatures were below zero °C.
For the other 2 references, I cycled them between -20°C and +80°C with decreasing temperature swing, and could bring both back to original value.
After excursion to low temperatures, the reference voltage will always go up by several ppm, and high temperatures will force it to lower values.
Quite obviously, you can't do anything in the external circuit to mitigate that effect.
There were several assumptions in the LTZ1000 thread, that the A type is less prone to this hysteresis, due to the different, assumed more flexible assembly of the chip to the case.
I've never seen any experimental proof or quantitative measurement for that assumption, so at current, this is simply incorrect... SN-18 also indicates, that the A version is affected as well.
This hysteresis effect is equivalent to the magnetization of iron. Therefore, it is not possible to use something like a 'burn in' process to eliminate this hysteresis for all times.
But as described in this patent, it's possible to temperature-cycle the chip to its 'virgin' state.
If you want low timely drift, you need to run the oven at 45..65°C. The 7000 reference, like the 732 A/B all run at 45°C.
Therefore, this Pickering patent does not work properly inside the 7000 references, as a symmetrical temperature swing of initial +/- 40°C cannot be accomplished.
Unfortunately, the rare owners of this device, like TiN, did not yet measure the real oven temperature, and also not the cycling profile.
Otherwise, we also could design this scheme into our existing references... as far as it's really working.
At current, it's only possible to log the temperature during transport, and to compare the reference value before and after shipping.
Hot shipping is the alternative method, with the known disadvantages and shortcomings.
Frank
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