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From: john larkin <JL@gct.com>
Newsgroups: sci.electronics.design
Subject: Re: Datasheets and probability
Date: Sun, 15 Dec 2024 07:56:06 -0800
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On Sun, 15 Dec 2024 13:08:02 +0000, Martin Brown
<'''newspam'''@nonad.co.uk> wrote:

>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.
>

I can imagine that production test and trim create a nonsymmetric
distribution of some spec, and chop off the tails. And make a hole in
the middle where premium parts have been binned away to sell at a
higher price.


>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.

That doesn't seem to be the pattern nowadays. We buy ultra-cheap 1%
resistors by the reel, and they are really 0.1% resistors. In a
production line, with laser trimming, seems like all the resistors are
right on. 

There's no reason to buy surface-mount resistors that are spec'd worse
than 1%. It's easier to design knowing all the various Rs in stock are
better than 1%. 

What we pay extra for is low tempco. And thoe are usually way better
than their spec. Tempco is a nuisance to test for, at the PPM level.

Caps are not usually laser trimmed so are not as good. And they have
plenty of other issues that work against accuracy.



>
>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.

Some part data sheets have histograms, which can hint at what you'll
see in real life. 

Abs max voltage ratings are especially interesting. Most silicon
mosfets seem to zener reliably at 1.2x abs max drain voltage. Some
have gate zeners, some don't. Some fets and RF parts are reliable at
2x abs max data sheet voltage.

RF schotttky detector diodes can have silly ratings like 2 volts max
reverse. If you need their absurdly low drop and capacitance, you've
got to test them and break the rules.