For a first order low-pass active amplifier, the capacitor is connected between the positive input of the op-amp and the ground.. Here, in the second order, one capacitor is connected to ground and the other is connected to the output of the op-amp as it's shown in the attached figure. My question is about C1, why it's connected to the output and not to the ground as C2??
Thanks

 

Two RC networks have a very droopy response.
A Butterworth 2nd-order lowpass filter supplies positive feedback to the C1 capacitor to boost the response at the cutoff frequency to make the total response flat up to the cutoff frequency then a sharp drop past the cutoff frequency.

 

Thanks for your quick reply

 

Take a look at this Application note on active filters.

The topology you have posted is the General Sallen Key Low Pass.

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