Electronics > Metrology

LM399 based 10 V reference

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--- Quote from: imo on September 30, 2021, 11:30:38 am ---I collected several MAC199 (LM399 clone in TO18 case) with broken heater. I've tried to utilize the parasitic diode for chip temperature measurement.

Basically it works, with say 80uA fw current the voltage at the heater's ground changes like (aprox)

31C (fingers)  620mV
26C (amb)     637mV
IPA large drop  677mV

There is an issue with it, however - while the 80uA current is injected into the heater's gnd (parasitic diode anode) the Vref (standard 10V Vref like the above) increases by +3000ppm. That is way more than a drop on the bond wire - thus it messes up something on the chip, it seems..

--- End quote ---
Bringing the substrate above circuit ground may forward bias parasitic diodes from the substrate to circuit elements like NPN collectors.
Try biasing the substrate normally and using the diode going from the substrate to H+ instead. (If the heater failure is not due to H+ being open circuit).

Hello all,

I've had supid idea:

I do not own a 7.5 or 8.5 digit multimeter, only 2 PREMAs and one HP 34401a, so all 6.5 digit types. These devices are using the same reference as my reference board that I'd like to characterize.

My thought was based on better thermal stability of LTZ1000  based reference design. I thought I will replace the LM399 in on of my PREMAs and put a LTZ1000 based reference in. The 7.2V could be direct used in the PREMA. Due to the higher stability of the LTZ100 I will now be able to see the drift of the LM399 based 10V reference, if the drift is > 10┬ÁV.

What do you think about this Idea?

Edit: may be I will use one of the new AD ADR1399 references for the 10V reference board, when I get some ...

Improve the stupid idea: Use the LTZ1000 as a standalone stable reference and take a 34401A to measure the voltage difference between a DUT like the LM399 and the LTZ1000 in opposition mode (ref-minus connected to ref-minus).
Dont really know if a LTZ1000-modded 34401A would show better stability or if its limited fundamentally by its ADC/Frontend.

I totally agree with Echo88, at least the Prema DMMs are not limited by the noise of the reference, but from the ADC. Using them in the 100 mV range to measure the difference between two references makes much more sense.



most LM399 references that I have have less than 2 ppm voltage change near room temperature over a 30 deg C span (10-40 deg C)
2ppm/30 deg C = 67 ppb/K.
A LTZ1000 is usually not better if it is not trimmed for T.C.
So you have a hen and egg problem.

Attached a measurement between 2 LM399 (one at room temperature and one in my car cooler box)

With best regards



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