Electronics > Metrology

Capacitors for precision circuits

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tszaboo:
I was designing precision stuff in the past, and there were a few bits, which required that little bit of extra precision for the capacitors. Becuase they affected the signal directly. I'm talking about TrueRMS circuits, RC filters for ppm level measurements, where the leakage currents would skew the output, these kind of applications. For these, my default go to solution was the Panasonic ECPU
Stacked Metallized Plastic Film Chip Capacitor
http://www.farnell.com/datasheets/1733019.pdf
Because it is SMD. And that was important at the time. There wasnt too many choices, but this one worked with the 1GOhm insulation resistance, and the ok AC characteristic up to a few khz.
But damn, these things are expensive if you buy only a few, and you dont care about the package. These will be connected to max 15V so X2 rating is not required.

So my questions are the following:
Do you have a preferred DC film capacitor brand?
Or do you have like a magic NP0 brand and series, which is working?

Thank you for the answers.

T3sl4co1l:
FYI, I've heard that those parts can leak: the stack isn't sealed on the sides, and is pretty much impossible to wash.

I think this was relevant to a S&H, but similar concerns apply at your ~1Gohm level.

PS was always the old favorite, and Teflon, glass, and other exotica of decreasing production volume and rapidly growing cost; but they're all pretty much unavailable these days.

Afraid I don't have any relevant experience, I haven't worked much with low leakage circuits.

Tim

daqq:

--- Quote ---FYI, I've heard that those parts can leak: the stack isn't sealed on the sides, and is pretty much impossible to wash.

--- End quote ---
That's a nice surprise :D


--- Quote ---Or do you have like a magic NP0 brand and series, which is working?
--- End quote ---
Dunno about NP0, but mica capacitors are pretty stable and precise.

Edwin G. Pettis:
Unfortunately some of the best film capacitor types are no longer with us.  About the best solution is a high quality polypropylene capacitor, you may have to try different manufacturers to find one that works for your application.  If the capacitance isn't too large, go with a non metalized polypropylene.  Depending on your value, you may be able to find some polystyrene NOS, these were discontinued a few years ago but there is still inventory around.  Most of the relatively reasonable priced caps are very similar in performance and most of the higher priced 'exotic' caps do not get you that much better performance for the cost.  You can find polypropylene caps in 50V or 63V ratings.  Stay away from the so-called audiophile caps, you're paying a lot for marketing hype.

Surface mount caps tend to have problems the leaded caps don't and if you need the best performance, forget SMD and no, ceramic caps are not on the approved list for this type of app.

The Soulman:
Wima fkp and fks series should do well, not small nor cheap tho.

(these days the crappiest website ever.  :-- )

https://www.wima.de/en/our-product-range/parametric-search/

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