# what frequencies do caps filter?

I've been trying to find out what frequencies get filtered out by which type and capacity capacitors, but cannot find simple rules that say "type X, cap Y filters xxx (k/M)Hz - yyy (k/M)Hz". Yet everything I read seems to indicate this is common knowledge - or does everyone just use trial and error?. Who can help me out in providing some clarity on the matter?

Similar with ferrite beads/chokes. I know a big hunking choke helped me filter out the interference in my circuit, and have my eyes on some SMD-sized beads for on my small IC. How do I know whether the big and the small choke both filter the same frequency? An answer that tells me how to read a datasheet to find this out for myself is perfectly acceptable btw, I have poured over many a datasheet in the last few months :)

• Is this for bypass caps ? – efox29 Nov 26 '14 at 12:41
• Look for frequency characterization of the parts or measure it yourself. Pay attention to the SRF (as outlined in LvW's answer). This is true for caps, chokes, ferrites, etc. – user6972 Nov 26 '14 at 18:31

A capacitor's impedance is frequency dependent.

$Z_c = \frac {1}{j \omega C}$

Capacitor impedance is inversely proportional to C.

If you look at a datasheet, you might see something like this (sometimes you have to dig through the manufacturers website)

If you look at the 100nF curve, you can see that as frequency increases, its impedance decreases. Fantanstic. But wait... It's rising again at 20-30Mhz. That's what we would expect from an inductor.

Well..

A real capacitor can be modeled as a series capacitor, resistor and inductor. That rise that you see at that frequency means that the parasitic inductance is now the dominant term.

If you have a capacitor strapped across your rails, if you get any high frequency noise in there, the capacitor will look like a pretty good path to ground since its impedance is so low relative to the load. Different capacitors can handle different frequency ranges but typically low value caps decouple/filter high frequency (eg 1nF curve above) and higher value caps decouple/filter lower frequencies (eg 100nF curve)

• Also different construction methods and materials change the SRF (self resonance frequency) considerably. – user6972 Nov 26 '14 at 18:29
• Materials and size as well. Good point. – efox29 Nov 26 '14 at 18:48

"I've been trying to find out what frequencies get filtered out by which type and capacity capacitors, but cannot find simple rules that say "type X, cap Y filters xxx (k/M)Hz - yyy (k/M)Hz"

Capacitors alone do not "filter". Only in conjunction with other parts (R or C or both) we can realize a filter operation. The basic principle is based on a frequency-dependent voltage divider. More than that, the output of such a voltage divider not only is frequency-dependent but shows - in addition - a frequency-dependent phase shift.

A more complex filter is composed of more than only one voltage divider and the superposition of several signals with different phase shifts results in different filter functions (lowpass, highpass, bandpass,..).

These filter functions are independent on the TYPE of capacitor (forgetting unwanted parasitic properties) - however, of course the capacitance plays a major role because it determines the divider ratio for a certain frequency.