[volt-nuts] LM399 selection test PCB

Lars Walenius lars.walenius at hotmail.com
Thu Nov 26 10:46:05 EST 2015

Nice board! Will give you a lot of experience. See forward if you have more to report. 

What 2ppm/degC resistors do you use? Do you have any previous experience with them? 

Like noise , jumps could have its own thread. I think I have seen 5-10uV jumps on most of my 10V refs based on buried zeners. On some it has been years between and some very often. For some I have only seen it during temperature scans over a span of a couple of °C. Sometimes at low temperature and sometimes at higher temperatures. Of course the LM399 is special with its fixed heater. On one DMM based on LM399 I can have minutes or sometimes hours with 1.5ppm shift in reading but when it goes back for a long time. This have been ongoing for 20 years for that particular DMM.


Från: Ian Johnston
Skickat: ‎onsdag‎ den ‎25‎ ‎november‎ ‎2015 ‎23‎:‎29
Till: Discussion of precise voltage measurement

Hi all,

Thought you guys might be interested in a small PCB I made in order I 
could test, select & check pre-age (if there is such a thing!) my LM399's.

10 channel.
Each channel has it's own chopper op-amp (self bias the LM399),
2ppm/degC tempco resistors throughout,
2x2 sockets for the LM399's (same footprint).
Test points for each channel.
Jumper link on the power rail to disable each channel individually 
(op-amp and heater power).
Holes around each LM399 footprint to help isolate any thermal stress.


So far it's working out great.......i've already discovered one of my 
LM399AH's (sitting in ch.4 in the photo) is somehow unstable. It's 
output jumps around by about 10uV's. Need to investigate........

The idea is that I can test, select and pre-age LM399AH's before 
soldering them into my target PCB's. I understand that soldering 
can/will reset any aging.......but I figured it might help.

I did a pre-test on those Ebay LM399's I bought and they seem to work 
ok.......still to properly evaluate.....but I do notice that the new 
ones's I bought from Digikey are all giving over 7Vdc, the Ebay ones all 
well under 7Vdc.



/"because it feels good to make stuff"/

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