Re: Datasheets and probability

On 12/30/2024 7:37 AM, Klaus Kragelund wrote:

On 16-12-2024 11:23, Chris Jones wrote:

On 15/12/2024 12:50 pm, John S wrote:

Hi, men -
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There are sometimes 3 columns on a datasheet which may contain min, nom, and max values. Like Vds for example.
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Is there any probability tacitly assigned to the values? I know that nominal is the average, and I assume that the value lies between the two middle one standard deviation points.
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1. Is that a valid assumption?
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2. Is the min between one and two standard deviations down from the mean?
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Thanks and I apologize for asking such a basic question in this professional group.
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Cheers,
John

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For anything that has been trimmed (whether or not you know that it has been trimmed, especially digitally trimmed), assuming that parts will have a gaussian distribution in parameters is a mistake. If the part can auto-calibrate itself in use, even more so.
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Also, batch-to-batch variation often exceeds part-to-part variation within a batch. They might not know the batch-to-batch variation at the time when they are writing the datasheet, as there is probably only one batch. They can run "skew lots" where they ask the fab to deliberately adjust the process parameters of some wafers to the upper and lower limits of some parameters, but this is never exhaustive, and so the limits in the datasheet will likely be sand-bagged (overly cautious) to some degree.
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Also, specs that are not a major selling point would likely be chosen to be very easy to meet, because they really do not want to be throwing away parts because of a spec that nobody cares about. For example, if you buy a low-noise amplifier, you might reasonably expect that the noise figure specification (which involves a trade-off with power and/or chip area) is chosen such that they can meet it on say 99% of the untested parts, so some of them will be only just passing by a margin equal to the uncertainly of the tester calibration. On the other hand, if there is a CMOS logic enable pin of the low-noise amplifier which typically has a leakage of a couple of femtoamps, it might be specced with a maximum leakage of 10 microamps, just because it would be stupid to throw out one of these amplifiers if the package was slightly dirty and leaked a picoamp instead of a femtoamp, and unreasonably expensive to configure the tester to be able to tell the difference.
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 A guy at EEVblog forum has done a lot of measurements on resistors:
 https://www.eevblog.com/forum/projects/smd-resistor-distributions/
 

Yes. Thanks, Klaus, I watched that some months ago.
And thanks to everyone for your comments. Cheers,
John S