Electronics > Metrology

LM399 with external oven

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I'm trying different approaches for a design i want to build, and I found asking myself: "Does it makes sense to use the LM399 without its heater but relying on an external oven?"

For use as a reference outside of an instrument it really doesn't make any sense to run the internal heater at a scorching 90C, something like a warm but comfortable 40-45C should be sufficient to reduce the output noise, heat the whole board and improve thermal isolation, so i want to try it.
However, before designing and ordering the PCBs, i wanted to ask if you have any experience with this, just to be sure if it's feasible. I'm just starting to work seriously with precision equipment and components, so i have many doubts, and my main doubt for this project is: "What if the LM399 is optimized to run at 90C and a lower but constant temperature makes things worse?"

It is an interesting question. A lower temperature could reduce the long time dirft and one can expect a little less noise, at least for the higher frequency part. For the 1/f noise part and popcorn noise things are not so easy: the higher temperature may well shift things to a higher frequency and this way get less noise at the very low frequency end that really hurts. The 1/f noise curve is only an approximation and the spectrum may actuially get flatter to very low frequencies.
Especially with the popcorn noise that is very common with LM399 more frequent jumps can be a good thing, even though the reference may than look more noisy.

A lower temperature may reduce the power needed, but the extra heater would add to the size and this tends to increase the power needed. So the reduction in power may not be that large.

In this case you can forget about the LMx99 and use LM129/329 instead. With a bit of luck you can find some NOS in the hermetic package somewhere.



--- Quote from: NaxFM on May 30, 2022, 05:32:27 pm --- "What if the LM399 is optimized to run at 90C and a lower but constant temperature makes things worse?"

--- End quote ---
Its most probably not. Otherwise we would not have such a large stray between unheated and heated zener voltage.
The best zero-TC voltage for a LM399 is near 6875 mV.


The other question is: how good can you thermally isolate a package which has much more size than the tiny TO-46 package?
I fear heater power wise you will not gain much.
And keep attention to the temperature sensor(s) of the oven. The LM399 sensor is not placed optimal so that you get output voltage variations depending on orientation of the LM399. (Tilting effect).

with best regards



--- Quote ---The best zero-TC voltage for a LM399 is near 6875 mV.
--- End quote ---

I'm still not convienced about that theory, as I remember also the following image of that diagram added with measurements I did on several specimen.



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