Electronics > Metrology

using an ICL7650S to buffer the output of an LM399

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robert.rozee:

--- Quote from: Andreas on April 12, 2024, 05:55:43 pm ---b) its time that you read the LM399 thread most of your questions are handled there:
https://www.eevblog.com/forum/metrology/lm399-based-10-v-reference/msg441913/#msg441913

--- End quote ---

i have read through the "LM399 based 10 V reference" thread a couple of times in the past, but the problem is that it contains too much information; with nearly 1400 postings many dozens of different approaches have been explored, analyzed, and numerous conclusions drawn in an attempt to wring every last ounce of performance out of the LM399. there have also been many paths that, while promising, have ultimately lead to no useful improvements.

whereas i'm just after something that is 'good enough' for a specific task, while fitting within a few non-onerous design criteria. and to this end, almost everyone who has contributed to the couple of dozen postings to my thread here has added valuable information in a form that i can make use of   :-+


cheers,
rob   :-)

robert.rozee:

--- Quote from: Andreas on April 12, 2024, 06:30:58 am ---I hope the 24V for the heater are stabilized. the LM399 is sensitive to heater voltage variations.
--- End quote ---

hi Andreas,
    i've been thinking about this one for the last few days. over in the LM399 thread, back in 2014, you made some measurements of this effect with the following results:



(copied from: https://www.eevblog.com/forum/metrology/lm399-based-10-v-reference/msg441913/#msg441913)

i've added in blue lines to mark off the region between 18v and 25v heater voltage (corresponding to 6x Li-Ion cells going from fully charged at 4.2v each, down to a 3.0v cutoff when fully discharged), and the corresponding change in zener voltage. in the setup you used at the time, this works out at about a 6uV drop after the 2:1 scaling, or 12uV directly at pins 1 and 2 of the LM399.

i presume that each run was conducted over a relatively short time interval? has anyone else repeated and verified this experiment since? i'm also wondering if there could be mechanisms other than the change in heater voltage coming into play; the 12uV shift in reference voltage corresponds to about 1.7ppm and thus is tantalizingly close to the 1ppm level that i'm trying to keep below.


cheers,
rob   :-)

Andreas:

--- Quote from: robert.rozee on April 17, 2024, 02:25:10 pm ---
i presume that each run was conducted over a relatively short time interval?


--- End quote ---
Hello,

each measurement point is usually 1 minute averaged reading in my measurements.
And it looks like 250 mV steps between each measurement.

And of course both directions (rising and falling heater voltage) to make thermal effects visible (if any).
Have done the same for LM399#14 and for the first four ADR1399 in LS8 package

Edit: why do you think there is footnote (Note 3) below the ELECTRICAL Characteristics table in the LM399 data sheet?
And another measurement (from macaba) :
https://www.eevblog.com/forum/metrology/adr1399-reference/msg4114465/#msg4114465

with best regards

Andreas

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