Hello,
Here is another update on this DIY voltage reference. Accommodating all changes (with many thanks for the kind suggestions!)
required a new build. I did not design a real PCB yet, as the new version may need some more changes,
and some parts of the new design were just not settled enough.
The part that got the most attention was the resistor divider. I had some decent (not top, but decent) precision resistors
that I had bought on some previous occasion. It was a mixed bag of two dozen or so single values.
Given that the LM199 does not give an accurate output voltage (stable yes, accurate not),
the actual values of these resistors did not matter very much. I chose them for their tempco and stability.
Just as an example: the set contained one resistor of 62.5 Ohm and one resistor of 62.6 Ohm.
As the LM199 gave a measured output of just over 7 Volt, I wanted to make a 3:7 resistor divider.
So I wrote a program that calculated all kinds of compositions of two or three resistors from the set.
After some selection stages, I ended up with two sets composed of three resistors connected in series,
that had a very good ratio, and whose sum was close enough to 10 K (R5 through R10 in the schematic).
After completing the build on another prototype board (with another helper board to emulate the Kelvin connections),
I measured the output being 20 mV over 10 V. With some trial and error, I added a 120K resistor in parallel
to the 7K compound resistor, and that brought the error down to just 1.3 mV over 10V.
That was the limit of what I could do with my DMM at home, and that is what is shown in the schematics.
I completed the buid with a repurposed metal box (it used to be a video switch) that miraculously had the right holes
on the front side for an on/off switch and two output connectors. I added two 9V batteries in series,
and thought about how to make a stable supply voltage somewhere between 12 and 15 Volt.
As mentioned previously, I had an LT1129 module that seemed to fit nicely, and so it was chosen.
The output voltage of the module was originally 10.2 Volts, so I increased it to 13 Volts.
The resulting build can be seen in the attached pictures.
The schematics show the build as measured on a Keithley DMM6500 and an Agilent 34401A.
The two meters do show some voltage differences, but within their specified accuracy.
Using these meters, again with some trial and error, I added a 6 MOhm resistor in parallel
to the already present 7K and 120K resistors (three 2M resistors in series). This increased accuracy
by another factor of 6, bringing down the output voltage to a comfortable 0.2 mV above 10 Volts.
So the end result is that I now have an accurate reference that I can use at home to calibrate my meters.
The resulting design does not use a trim pot, but relies only on a hard trimmed resistor divider.
The bootstrap configuration gives a very stable output, which can easily be loaded with 10s of mA.
Currently it still lives on a prototype board, and for now that is fine for me.
I do wonder how much the reference will drift over longer periods of time.
It is turned off most of the time, so drift will (hopefully) be slow.
Again many thanks for all helpful comments and suggestions!