[volt-nuts] LM399 selection test PCB

[Ian Johnston]

Hi,

Sasumu RG2012L-223-L-T05 from Digikey - 408-1636-1-ND
0805 package.
£2.90 each in low qty.
My first experience of them.......
I was also trying 5ppm/degC resistors but they drifted like hell.........
I haven't done the sums yet so either the 2ppm ones are good, or the 
5ppm ones aren't so good.......there was too much drift to simply be 2 
versus 5 ppm/degC.

The jumps/noise one the one LM399 is every second.......haven't 
investigated yet.

Ian.





On 26/11/2015 15:46, Lars Walenius wrote:
> 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.
>
> Lars
>
>
>
>
>
>
> 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.
>
> Here:-
> http://www.ianjohnston.com/temp/lm399select.jpg
>
> 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.
>
> Cheers!
>
> Ian.
>
>
>

-- 

Ian Johnston
www.ianjohnston.com
https://www.youtube.com/user/IanScottJohnston
/"Because it feels good to make stuff!"/





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