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DunkH 14th October 2008 06:58 PM

Are Class X2 caps a good choice for decoupling?
I have searched but cant find much.

I just want to stock up on basic values such as 0.1uf & 1uf for use in decoupling voltage regulators, op-amps and chip amps. Also as bypass caps for bigger electrolytics.

I have read quite a lot on different cap types and it seems that the best are polystyrene followed by polypropylene. However these are quite expensive.

I notice that class x2 caps are polypropylene and are fairly cheap. Would they be a good choice for decoupling purposes?

If not what would be a good step up from polyester caps? And where can I get them in the UK?

richie00boy 14th October 2008 07:40 PM

For decoupling you want ceramic. Poly types are better for signal path.

I've always found polyester to be fine in signal path though.

DunkH 14th October 2008 11:15 PM

That seems to go against what I have read so far. I have been using polyester caps for some years but thought it might be worth tracking something superior.

There are a couple of articles like this

that use measurements of distortion and recommend avoiding ceramic and polyester caps.

I suppose what I am really asking is for a supplier of decent good value polypropylene film caps.

richie00boy 15th October 2008 09:36 AM

Those tests are for signal path not decoupling, the requirements are completely different for the two applications.

DunkH 15th October 2008 06:23 PM

a quote from the article I linked to

One thing seems quite clear, however, and that is the simple fact that you cannot "work around" the distortion problem in ceramics. Our feeling is that they should simply be avoided anywhere near an audio signal path and probably just avoided altogether for audio. For example, some listening tests have indicated that they can produce audible distortion when used as supply bypasses, let alone coupling!

I think I might re-post this as a specific "where can I get" question as it seems politically charged!

TheSeekerr 15th October 2008 09:58 PM

It really depends on what values are needed - for values up to about 100pf, NP0 ceramics are nearly always what you want. For values higher than that, film caps are generally better, largely because it's basically impossible to get NP0 ceramics in suitable sizes.

So to directly answer your question: if you can get the the class x2 polyprop's cheaply, and don't think they are too large for your application, go ahead and stock up on values between 0.1uF and 2.2uF, and they should serve you quite well.

However, if you're trying to use them in other people's PCB's, you may find that the pin spacing of X2's is too large - for example, an 0.1uF X2 is usually 15mm, and many PCB's will only provide the 5 and 10mm spacings used on polyester box caps.

All of the above applies only to power supply related roles - for signal path applications, ceramics, electrolytics and tantalums should never be used (with the very occasional exception of feedback paths, in which case there is occasionally an application for a very small ceramics). Polypropylene does give a measurable improvement over polyester, although I personally find that it's usually inaudible. I've never been able to afford polystyrene or paper-in-oil or any of those fancy caps, so I can't comment on those.

DunkH 19th October 2008 01:30 AM

Thanks, I think I will try them. Thay seem a fairly safe halfway house between polyester and "normal" polypropylene capacitors. I'm always keen to use anything that is good value because it is mass produced.

Geek 19th October 2008 01:34 AM

X2 poly caps can be inductively or non-inductively wound. I have found the more expensive non-inductively wound ones good for bypass into at least the low MHz regions.


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