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Design >> Analog Design >> Calculation Of Zin of audio filter
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Message started by Giorgos on Nov 10^th, 2013, 3:57am

Title: Calculation Of Zin of audio filter
Post by Giorgos on Nov 10^th, 2013, 3:57am

Hi ! I want to calculate the Zinput of this circut. What i am doing with Capacitors?Also, the Zinput until which devices is? maybe until first OPA ?

My frequency is 20khz.

Many thanks

http://i1284.photobucket.com/albums/a578/fovos1/SISOFILTE_zps56bc4977.png

Title: Re: Calculation Of Zin of audio filter
Post by aaron_do on Nov 10^th, 2013, 5:12pm

Hi,

that's a Sallen-key low-pass filter. I'm guessing you're talking about the in-band Zin. In that case, you can treat the capacitors as open circuit. Your input impedance is just the parallel combination of the two biasing resistors (looks like R1 and R3 but its difficult to make out). Nearer to the 3dB frequency, your input impedance will reduce.

cheers,
Aaron