On 11/09/2014 01:50, Mike S wrote:
On 9/10/2014 7:00 PM, Tony wrote:
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?
Isn't 8ppm/SQRT(kHr) better than 20ppm/SQRT(kH)? Why would the latter
be more expensive? Or is it the difference between "typical" and
"guaranteed?"
I'm guessing that typical in this case means the one sigma value so the
three sigma value would be 24ppm. In any case three sigma still only
means 93.32% of parts come within that limit, or 6.7% exceed 24ppm, and
a few could be considerably worse. That compares to a guarantee that all
all parts meet 20ppm. This link:
http://www.gellerlabs.com/LM299AH-20_Case_Study.htm provided in
Andreas's response is very interesting:
"Certified Long Term Drift The National Semiconductor LM199AH-20,
LM299AH-20, and LM399AH-50 are ultra-stable Zener references specially
selected from the production runs of LM199AH, LM299AH, LM399AH and
tested to confirm a long-term stability of 20, 20, or 50 ppm per 1000
hours, respectively..."
So in this case they really do mean a guarantee. And I doubt that
individual testing came cheap. I say 'came' because I wonder if they
still 100% test the 20ppm parts or if they select them using some lower
costs means?
Tony H
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