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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