Why not NP0/C0G caps?

[gmikol]

In this thread over in "Chip Amps":

http://www.diyaudio.com/forums/showt...91#post1445191

Tom Gootee wrote:

I looks like you have not used decoupling capacitors for each power pin on each opamp. Consider them to be required, unless they later turn out to not be.

Connect a 0.1uF X7R (or probably at least not NPO or C0G) ceramic cap from as close as possible to each supply pin of each opamp, to power ground.

I thought I had all this down, but this but I must have missed something along the way. NP0 caps have better tempco, and better tolerance than X7R types, right? So why not use them for supply decoupling?

Tom...you said it, can you educate me?

--Greg

[Conrad Hoffman]

Can't speak for Tom, but IMO NP0 caps are nearly perfect. Almost no dielectric absorption, almost no dissipation factor, stable, etc. The only problem is they only come in small values. If you need something larger, a different type cap is more practical. One caveat- the very low DF translates into very high Q (1/DF). If you have a situation where lead inductance is significant, you can get ringing. For bypass caps right at the IC pins, this shouldn't be an issue. The bypass cap test written by Jim Williams of Linear Technology and published in their app notes is always a good read when thinking about this.

[peranders]

I think it's as usual a rumour. In many place (not all), ceramics are excellent. Recently I have worked with TPS61031 boost converter and tantalum was not very exciting but 4 x 47 uF X5R did create an excellent output voltage at 600 kHz switch frequency. Still, the tanalum caps was Vishay low ESR but the X5R was much better. Ceramics has many not so good properties but if you know the material, you know also where to use them.

[Nordic]

NP0=C0G>XR7

[BrianL]

10nF/50V C0G/NPO are available in 1210 SMT package from Kemet and Panasonic among others. I have not tried them for an audio op-amp bypass application, but I plan to on my next project.

[AndrewT]

I think what is being said here is that in some locations a perfect capacitor can be a bad thing. Check some of Eva's post for ringing evidence.
Adopting a less than perfect capacitor can give better performance if the correct imperfections are selected.
Gootee is suggesting that x7r has worked for him and could be worth trying.

[gmikol]

Originally posted by Conrad Hoffman
The bypass cap test written by Jim Williams of Linear Technology and published in their app notes is always a good read when thinking about this.

Hi Conrad--

Do you know the AN # from Linear on that? Just trying to save a few minutes searching.

--Greg

[gootee]

Hi Greg,

I just finally saw this thread. And I see that AndrewT 'beat me to the punch', yet again. :-)

I don't consider myself to be an expert. But I will try to explain a little bit more about what I was thinking, when I posted that.

Yes, just as Andrew has said, my suggestion was aimed at the mere possibilty that a cap that was 'too good' might cause a problem, possibly exciting unwanted high-frequency resonance modes, when paralleled with an electrolytic. Therefore, in the absence of sophisticated measurements and calculations, and assuming the goal is for it to work the first time and further experimentation is not desired, a 'lossier' cap might be a better choice (i.e. a better guess). At the time, I probably mainly had in mind the following thread: paralleling film caps with electrolytic caps . I probably wouldn't blindly suggest a film cap, there, either.

I do certainly agree that C0G (aka NPO) caps are really-great caps. I have liked them even more, ever since I read Bob Pease's article about dielectric absorption (aka 'soakage'). I use them a lot, but almost never for power supply decoupling, unless they're part of a snubber network (i.e. resistance added).

[Nordic]

OK, then we can found grounds to agree on...

I only remember haveing problems with them once, and that was a low dropout voltage reg if I remember correctly..

[gmikol]

Wow...that's a lot of information to digest, especially in that other thread. I don't have access to a 'scope, and had been using film caps for supply decoupling, without any obvious bad effects. Perhaps I will switch X7R's for that application, since that seems like it might bea genearlly-accepted better choice.

--Greg