[volt-nuts] LM399 Long term drift specification

[Andreas Jahn]

Hello,

many questions I will keep it short:

All ageing specs are "typical" if you want to have "guaranteed" values 
you will have to measure it over a reasonable time. (I recommend min 6 
months).
Every treatment (soldering, mechanical/temperature shock) of a reference 
may create a new ageing cycle with different slope.
So 100ppm/15 years outside of "lab conditions" (23 deg , constant 
humidity) is something that I would not guarantee without re-calibration.
Although typical drift of pre-aged + selected references will be in the 
1-2ppm/year range if properly treated.
Also its meaningless if you want to have LT or National (TI) parts since 
LT is the only manufacturer which still produces them.
With high demands you will also have to sort out the "noisy" references.

Some "typical" LM399 (all from NS) ageing data can be found on web:

http://www.gellerlabs.com/LM299AH-20_Case_Study.htm
http://www.eevblog.com/forum/projects/lm399-based-10-v-reference/msg478496/#msg478496

With best regards

Andreas

Am 11.09.2014 um 01:00 schrieb Tony:
> I've just noticed that TI and Linear's specs for 'Long Term Stability' 
> (typical) are different. TI state 20ppm/1000Hr while Linear state 
> 8ppm/SQRT(kHr). That's  a big difference - is this likely to be a real 
> difference or just specmanship?
>
> I note that Linear (in Note 4) also state that "Devices with maximum 
> guaranteed long-term stability of 20ppm/SQRT(kH) are available." 
> Presumably they would be a special order as there doesn't appear to be 
> a unique part no. Would they be likely to be much more expensive?
>
> Then on page 4 Linear show a graph of long term performance of 44 
> units (rather cheekily starting the graph at 2 months or approx 1500 
> hours!). To reproduce something approaching the mean curve using the 
> formulae (drift ppm/SQRT(kHr)) x SQRT(month * 24 * 30.5/1000), 
> requires me to use 2.2ppm/SQRT(kHr). That is way less than the typical 
> 8.5ppm value.
>
> To get a curve that resembles the 3-sigma curve requires a value of 
> 5.7ppm/SQRT(kHr) which is still better than that 8ppm typical figure.
>
> I'm not sure how to interpret this; what value would you use if you 
> were designing a reference that isn't going to be re-calibrated after 
> the initial calibration and you don't intend to burn in for several 
> months?
>
> Assuming the equipment is expected to have a 15 year life, operating 
> in a range of 0 to 40C, what maximum total drift would you be 
> comfortable specifying? I'd prefer it to be less than 100ppm, but that 
> would require a drift of < 9ppm/SQRT(kHr), but that assumes that the 
> SQRT(KHr) drift characteristic is valid for periods much longer than 
> 12 months.
>
> Are there any other references, at similiar or lower cost, that could 
> be reasonably guaranteed to have a total drift of < 100ppm after 15years?
>
> Is it reasonable to assume that there are some types of voltage 
> reference will always drift, albeit noisily, in one direction allowing 
> the original calibration to be offset to some extent to reduce the 
> maximum error over its lifetime?
>
> Having looked at several application notes and lots of datasheets, in 
> those that include graphs of drift over 1kHrs or so of several 
> 'typical' examples, I have not been able to see any meaningful 
> correlation between the specified typical 1k drift figures and the 
> graphs. Eg. in Linear's Design Note 229 (Don't Be Fooled By Voltage 
> Reference Long-Term Drift and Hysteresis" the graphs of drift for the 
> LT1461S8 and the LT1790SOT23 show very different drift after 1600 
> hours - in the range 50 to 130 for the former and approx -5 to +45 for 
> the latter, yet the LT1461 is spec'd at 60ppm/SQRT(1kHr) and the 
> LT17910 at 50ppm.
>
> I realise that I would probably need to contact the manufacturers for 
> real answers but its been my experience that they aren't often 
> interested if you're not buying large volumes, and I know that a lot 
> of people here have a lot of experience in this area.
>
> Thanks,
>   Tony H
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