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From: John R Walliker <jrwalliker@gmail.com>
Newsgroups: sci.electronics.design
Subject: Re: Datasheets and probability
Date: Sun, 15 Dec 2024 12:44:40 +0000
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On 15/12/2024 12:11, Uwe Bonnes wrote:
> John S <Sophi.2@invalid.org> wrote:
>> Hi, men -
>>
>> There are sometimes 3 columns on a datasheet which may contain min, nom,
>> and max values. Like Vds for example.
>>
>> 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.
>>
>> 1. Is that a valid assumption?
>>
>> 2. Is the min between one and two standard deviations down from the mean?
>>
>> Thanks and I apologize for asking such a basic question in this
>> professional group.
>>
> Parameters often have a gaussian distribution, You then can expect typ
> values as the center of the bell curve and min/max values are cut
> offs. With cut off at 1 sigma, about 35 % of the part would be out of
> tolerance, so cut off is at a higher sigma value, but manufacturers
> will not tell you ar what value it is.

In the case of some TI op-amps there are different grades for parameters
like offset voltage.  They mark the package of the higher spec versions
before testing them and then throw away any that don't meet that higher
spec.  I was told this by a TI applications engineer.
Apparently it is cheaper to throw away a few op-amps than to have
branches in the production line to cope with different grades.
This does suggest that most devices are much closer to the typical
values than one might expect from the limiting values.
For parameters that take a long time to test the typical values may
be much better than the limit values.
John