Ceramic v. Film - Small Value Capacitors

[playpunk]

I am trying to come back from the pedalbuilding dead, and get some new builds going!

One thing I have been thinking about is the (relative) inconsistency of small value ceramic capacitors. I was poking around on mouser today, and found that WIMA has small value film caps that should work in most builds. Not cheap, but 5% tolerance instead of 25%.

Worth it? Has anyone ever actually tested the effect of capacitors on low voltage circuits? Will my crystal lattices be shinier with all fancy caps?

[m-Kresol]

Worth it? Has anyone ever actually tested the effect of capacitors on low voltage circuits? Will my crystal lattices be shinier with all fancy caps?

Well, film caps don't have a crystal lattice at all, if I'm not mistaken.

On a more serious note, I think it is depending on the application, but I guess in most cases it won't be worth the extra cost. Also, the leg spacing is different; hacking in a film cap might be a pita as well

[Govmnt_Lacky]

The only place I get picky about small value caps is in timing/clock circuits (a'la Chorus, Flanger, etc)

I'll use a silver mica or even film cap with tight tolerance in these instances. Otherwise.... its ceramics all day baby! 

[EBRAddict]

One thing to be aware of is the (relatively) severe voltage derating on some ceramic caps. That means that as the voltage applied across the cap increases the capacitance decreases. In some instances like X5R/X7R and cheaper it can be up to 80% at full rated DC.

I wouldn't use one in an audio filter, it might be hard to figure out what capacitance you're achieving without doing some spectrum analysis.

[gtr2]

Just use MLCC's

[Betty Wont]

Just use MLCC's

This!

[pickdropper]

One thing to be aware of is the (relatively) severe voltage derating on some ceramic caps. That means that as the voltage applied across the cap increases the capacitance decreases. In some instances like X5R/X7R and cheaper it can be up to 80% at full rated DC.

I wouldn't use one in an audio filter, it might be hard to figure out what capacitance you're achieving without doing some spectrum analysis.

For the really small value stuff, you can use NP0/C0G MLCC caps, which aren't voltage or temperature sensitive.  They get impractical (size and cost) with the larger value caps

For the larger value caps, X5/X7 are often acceptable compromises but, as you pointed out, they do have some voltage and temperature sensitivity.  The +80/-20% tolerance caps aren't X5/X7, though.  Those are usually Z5U or worse.  Generally, those shouldn't be put in the audio path.  Most designers use them for less sensitive applications where cost is paramount.

[EBRAddict]

Check out Page 4 of this PDF, as an example of voltage derating of X5R at -80% at full voltage.

http://www.niccomp.com/help/VoltageCoefficientofCapacitors-032012-R1.pdf

Granted that's for a tiny 0402 SMD, so it won't always be an issue but it's something to be aware of.

[pickdropper]

Check out Page 4 of this PDF, as an example of voltage derating of X5R at -80% at full voltage.

http://www.niccomp.com/help/VoltageCoefficientofCapacitors-032012-R1.pdf

Granted that's for a tiny 0402 SMD, so it won't always be an issue but it's something to be aware of.

Wow, those are some crappy X5R an X7R caps.  While far from perfect, usually X5R and X7R perform better (sometimes significantly better) than Y5V and Z5U caps.  The links below are an interesting read:

http://www.kemet.com/Lists/TechnicalArticles/Attachments/191/Why%2047%20uF%20capacitor%20drops%20to%2037%20uF-%2030%20uF-%20or%20lower.pdf

http://www.johansondielectrics.com/basics-of-ceramic-chip-capacitors

Generally, when buying MLCCs, it's better not to use them close to their rated voltage.