Choosing capacitor type

[Pha5e]

Hello,

I am a little confused on the type of capacitor to use in a precision Op-Amp transimpedance amplifier circuit.

As I understand, electrolytics are generally for supply decoupling and ceramics are used to supress high frequency noise.

What type should I use in the Op-Amp circuit to maintain measurement precision?

Thanks.

 

[godfreyl]

Among the more common types:

Good: Polystyrene or Polypropylene
OK: Polyester / Mylar
Bad: Ceramic, Tantalum and aluminum electrolytic.

Exception: NPO ceramic caps are good, but look the same as normal ceramic caps, which are lousy.

 

[Pha5e]

Thanks for your reply Godfrey.

I just did some reading on NPO capacitors and it seems they are highly stable even with temperature fluctuations. The only factor is the size of them.

What are silver mica capacitors like for stability/precision?

 

[godfreyl]

What are silver mica capacitors like for stability/precision?

I wish I had a clue, but I don't. :sad: I used to be under the impression they were good. Then I saw a couple of rude comments about them in another forum. Now I'm not so sure. Frustrating since I have no idea if there was any validity to the criticism or not.

As previously mentioned, I'd recommend Polystyrene or Polypropylene. Apparently Teflon caps are very good too, but not very common AFAIK.

...highly stable even with temperature fluctuations.

Other things to worry about are dielectric absorption and non-linearity. Some graphs I've seen of capacitance vs applied DC voltage for ceramic caps are quite horrifying - e.g. a change in capacitance by a factor of about 2 to 1 within the specified voltage range.