Problem designing a Sallen Key Low-pass filter

[mloven]

I wonder if any kind people can help me with this, I have never made any LP filter before, so some feedback would be appreciated:

I have a PWM signal that I can vary in frequency and I have chosen 10 kHz, 0-5 voltage. Then I want to put a second order SallenKey Low-Pass filter to the PWM so the signal filters.

My problem is to dimension the LP-filter and to get it to work. I’ve used this page from Analog device:**broken link removed**
and chosen: Fc to 10000 and “Gaussian to 12 dB” only for the convenience to get the capacitors to 94nF and 47nF. Then I have made the circuit and tested it but it doesn’t seems to work.

When I make a sweep from a function generator and trying different frequencies, nothing happens.

Am I missing something with the design? Are there any better method to filter the signal?

 

[Roff]

What do you mean when you say nothing happens? Do you get no output, or just no filtering? If you try to filter a 10kHz square wave with a 10kHz cutoff filter, you will just get a square wave with slow rise and fall times. What are you going to do with the resulting filtered output?