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From: Martin Brown <'''newspam'''@nonad.co.uk>
Newsgroups: sci.electronics.design
Subject: Re: Datasheets and probability
Date: Sun, 15 Dec 2024 13:08:02 +0000
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On 15/12/2024 01:50, John S 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.

It is actually a rather deep and difficult to answer question in general 
because for some components the answer can be "it depends".

Semiconductors I generally take it to mean ~3 sigma either side of the 
mean but design with a bit of extra margin so the 0.5% tail doesn't 
cause trouble.

But for some components like resistors and capacitors that may be 
obtained in 10%, 5%, 2%, 1%, 0.1% tolerances you can find that the 
frequency distribution of the components in the wider tolerance bins 
consists of values that are almost *never* inside the narrower ones.
IOW you are guaranteed at least 2% error in the 5% parts.

It is a bit better today than it used to be when they made batches and 
then selected from the process output. These days it is all a lot more 
reproducible and laser trimmed for precision parts.

-- 
Martin Brown