Re: Simple but precise ramp/triangle generator

From: Joerg (notthisjoergsch_at_removethispacbell.net)
Date: 09/15/04 

Date: Wed, 15 Sep 2004 22:56:39 GMT

Hi Rich,

>Just a nitpick, but does a ceramic cap actually vary, or is it just
>that with their wide tolerance, your absolute freq. could be off
>from design freq, which, of course would have some "variance" from
>unit to unit.
>
>
They have a high tolerance because except NPO they aren't meant for
frequency critical applications. The X7R market is bypassing and
coupling where capacitance doesn't matter much. So it's hard to get,
say, a 2% X7R cap. In NPO you can but that may turn into a nightmare for
the purchasing department unless they buy several years worth. It
wouldn't be the first time that a certain value is on back order and
nobody knows when you can get some again. Unless you can design
everything around 100pF, 1000pF and maybe 470pF.

>Or is an individual ceramic cap really unstable? I know they're leaky,
>and have crap TC, and +70-20 tolerances, but that's what makes them
>good bypasses.[0] :-)
>
>
As the other posters said, I also found drift to be an issue with
non-NPO ceramics.

Regards, Joerg

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