Input Impedence Of Mfb Low Pass Filters

[sean87]

Hi all,

I am designing a filter according to this guide: http://www.cirrus.com/en/pubs/appNote/AN048Rev2.pdf

On the second step, I should calculate the imput impedance of the filter (in symbolic way):



The transfer happens to be:



So far I tried as follow, but I am not sure if this is right nor I can simplify the equation more.


http://img820.imageshack.us/img820/1704/writingsh.png

This post has been edited by sean87 on February 11, 2012 08:14 pm

[Sch3mat1c]

My brain isn't following the math at the moment but it seems reasonable.

The series resistor might suggest an overall resistive response, or R+C given the filter stuff after it, but the op-amp being active and feeding back phase-shifted stuff could reduce that quite a bit (a double phase shift looks like negative resistance). So in general, the response could be ugly, which is why these circuits work easiest when you just say... to hell with it, drive it with a follower and forget about it!

Passive filters are designed to operate with ohmic sources and loads, but they don't present nearly an ohmic response outside of the passband either (essentially reflecting the out-of-band power).

Tim