[sdiy] Sonic qualities of parts..

[Tim Stinchcombe]

> I would add a further caution to X7R/Y5V in that the capacitance  
> varies quite considerably with DC bias, to the extent that you could  
> end up with 10-20% of the original value.  This effect is a right  
> pain when selecting decoupling capacitors, as you can be caught out  
> ending up with insufficient decoupling capacitance.

I'll second that, and add that some ceramic cap types show quite a lot of
variation with the applied _AC_ value too. Whilst investigating a strange
problem at work recently I had occasion to measure a 10uF, 0805 X5R ceramic
cap (so big value, small package, high-K dielectric), with differing applied
AC and DC bias. The results were hard to believe at first, but once you find
charts like those in the link Neil posted you do tend to start believing
them. Here is a post from the thread at Agilent's forum covering the whole
thing - scroll down a little for the two plots I made:

https://forums.tm.agilent.com/phpbb2/viewtopic.php?t=2815#8195

(I'm fairly sure this is 'public', and doesn't need an account to see - at
least it is letting me, not logged in, whilst at home, not work.)

One of the Agilent replies also posted a document from AVX, showing curves
of cap value variation with applied voltage, similar to that Neil linked:

http://www.avx.com/docs/Catalogs/cdesc.pdf

So my tuppence worth: caps can be a bloody minefield! But stick to the
fairly 'normal' stuff, and hopefully you don't get your fingers burnt too
often...

Tim
__________________________________________________________
Tim Stinchcombe 

Cheltenham, Glos, UK
email: tim102 at tstinchcombe.freeserve.co.uk
www.timstinchcombe.co.uk