Hello,
the LM385 (or derivates e.g. LT1004) are used in some portable DMMs.
It is specced with 20 ppm/kHr typical.
But usually this parameter is only valid for the nowadays obsolete metal can housing.
Coincidently I found this yesterday, reading Linear Technology's AN82-2 which states:
Most of the long-term stability figures shown in LTC reference data sheets are for devices in metal can packages, where assembly and package stresses are minimized. You can expect somewhat less performance for the same reference in a plastic package.Looking at various datasheets though LT mostly seem to specify which package the long term stability figures apply to. The LT1027 datasheet clearly shows the metal can package as obsolete but only quotes the stability figure for that package - presumably they don't think it's worth the cost of characterising the plastic packaged versions.
The LT1004 and LM385 datasheets though don't state the package so as you say its almost certainly the obsolete metal can part. I expect its a specification that's just been copied across from the old LM385 datasheet and the newer plastic packaged parts have probably not been characterised - hardly worth doing for $0.2 parts.
As Bob Pease stated in his musings "What’s All This Long-Term Stability Stuff, Anyhow?"
http://electronicdesign.com/archive/what-s-all-long-term-stability-stuff-anyhow"This usually makes the customer happy and/or shuts him up because (a) it is usually true, or close to true; and (b) if he wanted to get data, it’s a lot of work for him! And he doesn’t usually come back quickly. If at all."Deliberately quoted slightly out of context (he was talking about long term extrapolation of the 1000hr figures) - but I think it sums it up nicely. My guess is that anyone who really cares about long term stability will do their own testing - those that don't aren't going to know if the datasheet figures are remotely accurate. In any case they are almost invariably 'typical' figures so they aren't guaranteed anyway.
An example where the figures can't be trusted is TI's REF5050-Q1 (the automotive qualified part) datasheet. The headlined features include:
EXCELLENT LONG-TERM STABILITY: – 5ppm/1000 hr (typ) after 1000 hours[/b]
Wow. Remarkable for a $1.6 part. But further down it quotes:
LONG-TERM STABILITY (SO-8 parts):
0 to 1000 hours 90 ppm/1000 hr
1000 to 2000 hours 10 ppm/1000 hr
Hmm so not 5ppm/1k hr but still excellent. But most interesting of all there are some graphs (figs 22 to 24) showing the results of testing 96 samples over 2000hrs. Really useful - if only all datasheets published this information.
But It seems it was a mistake. The earlier revisions of the datasheet for the standard version of the (not automotive) REF5050 was exactly as above. The latest revision however states the long term stability as:
0 to 1000 hours 100 ppm/1000 hr
1000 to 2000 hours 50 ppm/1000 hr
Ok. A lot more credible. Also the stability graphs (figs 22 to 27) are now totally different. So almost certainly a mistake and the automotive part datasheet didn't get updated when the standard part datasheet was changed.
With plastic housings you will have additional humidity drifts.
On the other side: with a T.C. of typ 80 to max 150 ppm/K a 20ppm parameter is difficult to measure.
With best regards
Andreas
True. But the datasheets for many references show that the temperature coefficient is typically 0, or near 0 at around room temperature, increasing either side so perhaps not so difficult.