# capacitive filter design

I have a question about output capacitor filters.

So I know that a simple capacitor filter would have a single pole response, and this is just a first order filter. Let's say that the value of the capacitor here is C.

If I had split this single capacitor into 5 smaller ones, i.e. each capacitor has a value of C/5, would my filter now be 5th order? Would I also have 5 ESR zeros because of this configuration?

EDIT: here is an explaination of what I'm thinking

Given that the single pole RC filter looks like this: (source: buchanan1.net)

Would splitting that capacitor into 5 capacitors not increase the number of poles and zeroes in my system? assuming that the resistance in the wires is even 1 Ohm, would that not make multiple RC filters, and thus, increasing the dB/decades of my filter?

• In series? If so, no, the pole-zero plot does not change. More details would be helpful. – Matt Young Oct 1 '12 at 1:54
• Ah, I should have added that my capacitors are placed in parallel. – suzu Oct 1 '12 at 2:07

## 2 Answers

No matter the configuration, with 5 smaller capacitors, the circuit remains 1st order. However, you can get second order filters using capacitors alone with a Sallen-Key circuit. They can be chained together in series to make even higher order filters. The main advantage is being able to eliminate inductors from the filter entirely, but the circuit will oscillate if brought close enough to the complex axis.

• Upvoted, and here is a nice Sallen-Key calculator I use these days that some Japanese folk have put online. Just fill in the two R's and two C's and it makes nice graphs. sim.okawa-denshi.jp/en/OPseikiLowkeisan.htm (this is the low-pass filter: there are others). – Kaz Oct 1 '12 at 3:25
• That is nice, bookmarked. – Matt Young Oct 1 '12 at 17:59

Five capacitors wired in parallel of value C/5 are equivalent to a single capacitor of value C.