while working on a project i needed a really stable voltage reference so i made my self LM399 based 0.5 ppm/ C 10V refernce.
LM399 is nice shunt reference chip , Although initial accuracy is only 2% but accurancy does not matter , what matter is stability. Because i have recently calibrated Agilent 34465A 6.5 digit multimeter , i can easily find out absolute value.
With 0.5 ppm/C and 10V range , its translate to 5uV /C. At this range of temperature stability it is way beyond what my Agilent 34465A can measure.