Hello everyone,
Galaxyrise, nice set of curves, good repeatability within your limits. I have found one note that Bob Pease sent me so far, you may have a hard time reading his hen scratch, this was part of a letter he sent me back in 2003, he did not mention just what meters were involved in the test nor the exact date of the test. The 'graph's (I'll use that term loosely) indicated a repeatable drift in the measurements of the LM399 (he did not mention what the suffix was, an H or AH). The test was over some months long but Bob didn't mention just how long, usually a long term drift measurement was about 10-12 months.
The maximum TCV of a LM399H is 2 PPM/°C, if you are seeing anything higher than this, there is a problem with the test setup. The LMx99 is very linear, regardless of operating current, the internal temperature is regulated at ~90°C. The operating ambient temperature range for the LM399/A is 0°C to +70°C. The internal temperature of the LMx99/A is self regulating within the specified ambient temperature range. Your measurements should indicate a linear TCV with change in ambient temperature. All those curves seem to indicate problems with the test setup, even with repeatability of the curves, it indicates the error sources are relatively stable themselves. Yes, the LMx99/A run at 90°C so you are going to see heat radiating off of them, the zener generates much less self-heating as is the design intention. The heater runs at about 300mW at 30V, the zener would run at ~70mW at 10mA, at 1mA only ~7mW. The heater compensates for the zener's heat as well, mostly leaving just the TCV of the zener itself. The drift spec indicates a zener current of 1mA +/- 10%. I have used the LMx99/As and I have never seen anything beyond a consistent TCV with them. With varying ambient temperature, I have measured nothing more than the drift of the zener, usually under 1 PPM/°C for a LM399/A version. Thermal EMFs can be really difficult to control and they tend to be nonlinear in nature.
I have attached Bob's note to me plus a National data sheet from 1999 which gives some interesting specs. I also attached the LM199 data sheet from Linear Tech from 1990. Linear made some improvement in the TCV over the older National part, but we're talking a few tenths of a PPM/°C here.
I remember someone mentioning the old 1N827/A voltage reference zener which dates back to the 1950s from Motorola, this special zener could exhibit a TCV of 5 PPM/°C but this wasn't the only Motorola zener that could do that; the 1N4569/A, 1N4574/A, 1N4579/A, 1N4584/A, 1N4779/A, 1N4784/A, 1N939/A/B, 1N4769/A, 1N4774/A, 1N2624/A/B, 1N2169/A, 1N2170/A, 1N2171A and the 1N945/A/B. These zeners all had 5 PPM/°C TCV and voltages between 6.2V and 11.7 V nominally. The zener currents varied between 0.5mA and 10mA for best performance and the temperature extremes could be as wide as -55°C and +150°C, pretty impressive I'd say.