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Author Topic: Capacitors for Active filters  (Read 1998 times)
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technovm
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« on: January 21, 2011, 08:28:22 08:28 »

Which type of capacitors should be used for active filters and why?

Thanks in advance.
« Last Edit: January 21, 2011, 08:34:07 08:34 by technovm » Logged
LabVIEWguru
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« Reply #1 on: January 21, 2011, 05:39:00 17:39 »

Well, you would want to use low tolerance caps, assuming you want a tight filter. I was just looking at some caps on Mouser that listed their tolerance as +/- 20%. When you plug those extremes into your equation you could get a filter that isn't operating within your desired window. Temperature compensation / temperature stability is another concern. The value on your caps (and resistors) is going to change at a certain rate as the temperature changes. Silver Mica caps were the top choice a few years ago, but, man, talk about expen$ive! High end (more money than sense) audio equipment and a lot of medical equipment uses Silver Mica caps in the filters. They are the ultimate. Resistor values are then calculated around the values of the caps.

Depending on your application, you might look at active devices like switched capacitor filters, or Maxim makes a device (I haven't played with one yet) that is an active filter IC that allows you to set the filter type, feed it a clock (calculated on freq and type of filter) and you are done.

A lot of years ago I built a teletype (rtty) interface for radio that used MF10CN switched cap filters. They were really amazing at the time, and I imagine they've got devices that really improved on that device by this time. I seem to remember polystyrene (?) caps were real popular for a while, they had a lot of the same characteristics as Silver Mica, but as they aged their values turned to trash & the equipment quit working. (If I've got that wrong, someone correct me.)

You've stumbled into a subject that is almost a religion with some people. With all that said, I guess that is why DSP is such a hit.
« Last Edit: January 21, 2011, 05:55:43 17:55 by LabVIEWguru » Logged
Triodethom
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« Reply #2 on: January 21, 2011, 11:51:59 23:51 »

My luck with Polystyrene caps is they are damaged by temp and  over-voltage .  The high-end (money over reason ) is now more likely to use Teflon cap  very expensive  but very stable and close to ideal for a large price.  Silver Mica looks like  a cost effective choose next to Teflon  . I must agree with LabVIEWguru about polystyrene  cap aging badly  also it is a religion for some people.   If your budget is big go Teflon closest to ideal made period. 
Is this an active analog or digital filter ? That dictates what material the resistors should be made of.
 Most often  cost changes what can be used for passive parts in a filter.

Hope this helps.     
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Signal
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« Reply #3 on: June 30, 2014, 03:53:42 15:53 »

Tolerance and stability as important factors for filters were mentioned. Also do not forget about linearity. The main source of non-linearity in case of capacitors is voltage dependency (secondary - "microphonic" piezo effect). That is why all ceramic caps except NP0(C0G) type are not used in any signal path including filters if %% of distortions are not acceptable. In remainder - different kinds of film capacitors.
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Gallymimu
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« Reply #4 on: July 01, 2014, 04:22:10 04:22 »

You can use whatever you want.  You have to provide more information!!!

If you are just making an antialiasing filter any cheap ceramic cap will be fine.  If you need a precision audio filter then the answer is different.

In general be wary of X7R and Y5U or X5R as their capacitance has a heavy voltage dependance that exceeds the tolerance ratings.

C0G and NPO caps are pretty darn good but you pay for them and they don't have the capacitance/volume of the other cheaper ceramics.
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solutions
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« Reply #5 on: July 01, 2014, 05:34:54 05:34 »

Nothing like cracking open a three and a half year old topic, guys.

http://bit.ly/1m4gq68
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bbarney
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Uhm? where did pickit put my mute button


« Reply #6 on: July 01, 2014, 09:38:31 09:38 »

And here I thought I was the only one here who ever looks at the dates on the posts  Roll Eyes 
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