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The LM399 datasheet specifies a temperature coefficient "\$\Delta\$T/\$\Delta\$Temp". The conditions state an ambient range but don't specify if that's with or without the heater on.

Which is it?

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  • \$\begingroup\$ Where the Temperature Coefficient goes, I see dV/dT, but no dT/dT. Whatever it is, T is the temperature that the Zener sees, doesn't matter with or without the heat from internal or external heater. BTW, could you answer, please, this one, if you know it? \$\endgroup\$
    – jay
    Nov 25, 2021 at 23:10

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If you read the Note 3: "These specifications apply for 30V applied to the temperature stabilizer and –55°C ≤ TA ≤ 125°C for the LM199; and 0°C ≤ TA ≤ 70°C for the LM399."

Means that the heater is ON and powered through 30V. If you use different heating power, precision might vary.

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  • \$\begingroup\$ Ah! I totally missed that note 3 applied to the table on the previous page too! \$\endgroup\$ Nov 26, 2021 at 5:44

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