Hi 3roomlab, :-)
My opinion is that you really can not make a cheap voltage reference, with reasonable specifications.
Everyone should stop with the word "cheap" and voltage reference in a sentence.
You are just kidding yourself...
I do a lot of testing on references but every time, the way you built the reference, is really very important.
Its a big part of the stability!!! and remember not to build the reference in a plastic box, METAL is wat you want.
I can create beautiful schematics but if the quality of the components and construction is not good, then it will never be a good reference.
It has already been mentioned many times on this forum, that testing of references is expensive.
If i send away my HP 3458A for Cal/Adjustment, its about 900$, for almost that price i can buy a KEYSIGHT 34461a :-)
And my 3458A is NOT 24/7 on, thats also expensive, i switch on the 3458A 24 hour before i start measuring...
Most of my daily testing is done, with TEK4050 en 34461 DMM's.
Last week i tested some LM399 schematics, the sensitivity for battery voltage change and the power that's needed for the LM399 heater.
My starting point was three 9V batteries in series, to keep the current for LM399 for the heater low.
The circuit does not start, because when you turn the circuit on, the heater current is really high.
If your batteries are getting older, the Ri is also higher, and the batteries cant deliver the peakpower that is necessary to start the heater.
Problem, problems, problems,
And now some pictures, poluting this topic
These are the components i use, for the testing i do, with The LM399AH references.
The resitors are Rhopoint 8E16D, Copper band 0.1mm and Vishay 15PPM trimmer pots (1 piece 2x price of a LM399AH!)
7 Resistors, the 4K leg and 10K leg, maybe i put some 2 component glue in it to seal it a little.
The copper band keeps the temperatuur of the resistor nice close together, but i have to test it first, to see if its realy works!
Vishay 1280G trimmer
I'am going back to the LAB bench, for testing of the LT1236-5V LS8.
I do temp sweeps between 42,5C en 29C, most of them, are well within the specifications.
Kind regarts,
Blackdog