[volt-nuts] any unheated Vref better than LT1021-7 (at 7 ppm/khr typ.) ?

beale beale at bealecorner.com
Tue Jan 1 17:50:21 UTC 2013

Happy 2013 to all volt-nuts! 

I have a question about unheated voltage reference chips. I am interested in long-term drift, for example ppm in the first 1000 hours.  The manufacturers seem coy about this spec, as I have not found any parts listing allowing you to sort on that parameter.

The lowest drift spec I have found so far is the Linear Tech LT1021-7 which claims 7 ppm for the first 1000 hours (typical), although the data sheet does not provide a graph. The -5 and -10 volt versions of the same part are twice as bad, attributed to the internal output scaling resistors. I have a few questions:

1) Does anyone know of an unheated reference with a lower 1khr drift? Not counting the "deliberate lie" that http://cds.linear.com/docs/Design%20Note/dn229f.pdf complains about.

2) I have seen mention of dipping a PCB in molten paraffin as a conformal coat. Can anyone say if that is hermetic enough to remove the "typically 100 ppm" drift from relative humidity changes as mentioned on p.13 of the LT6655 datasheet http://cds.linear.com/docs/Datasheet/6655fb.pdf ?  I gather the board does have to be scrupulously clean before the coating.

3) If the humidity effect is strictly from mechanical stress on the die as the plastic encapsulant swells or shrinks, would a part in a metal can then be immune from this effect, even if it was non-hermetic?  (Assuming that it was also mechanically decoupled from dimensional changes in the PCB.)

Thanks for any insight on these issues!

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