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Posted: February 11, 2012 08:13 pm
Member No.: 34,251
Joined: January 12, 2011
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.
This post has been edited by sean87 on February 11, 2012 08:14 pm
Posted: February 11, 2012 08:58 pm
Forum Addict ++
Member No.: 73
Joined: July 24, 2002
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).
Answering questions is a tricky subject to practice. Not due to the difficulty of formulating or locating answers, but due to the human inability of asking the right questions; a skill that, were one to possess, would put them in the "answering" category.
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