I want to use a second order active low pass filter to cut off frequencies over 33kHz . The input is 1.3 Volts peak to peak sine wave of 32 Khz and I want to make it 30kHz of 1Vpp . The formulas for calculating the values on the components of S.O.A.L.P. are for different SOALP than mine . Can someone provide me with the formulas that I need to calculate the values of my schematic or better provide the values and explain

Edit: the op amp is provided 20V in the [+] wire and is grounded on the [-] wire

Edit 2: the formulas I found for other SOALP are on this link http://www.electronics-tutorials.ws/filter/second-order-filters.html

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  • \$\begingroup\$ Are you sure this schematic you have given will work as a Second order low pass filter ? \$\endgroup\$ – Mitu Raj Dec 29 '17 at 15:34
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    \$\begingroup\$ @MITURAJ that is actually a classic second-order Sallen-Key LPF \$\endgroup\$ – altai Dec 29 '17 at 16:05
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    \$\begingroup\$ @MITURAJ oh this will help a lot with the calculations thank you \$\endgroup\$ – maverick98 Dec 29 '17 at 16:10
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    \$\begingroup\$ @MITURAJ great ref. used it too many times ! \$\endgroup\$ – altai Dec 29 '17 at 16:10
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    \$\begingroup\$ No linear filter will convert a 33 kHz input signal to a 30 kHz output signal. \$\endgroup\$ – The Photon Dec 29 '17 at 16:31

There are better steps to design a filter of this type;

Define specs then choose the type of knee response.. Low Q like Bessel give flat group delay or rate of change of phase shift. Higher Q with steeper edge like Chebychev compromise group delay but steeper edge and some ripple in passband. Others include Butterworth, etc.

Filter specs

  1. Choose Type & order (2, 4, 6, 8 ...) (low cost and much better performance)
    or choose characteristics ( better)

  2. Ensure DC bias is in valid CM range by adding biasing, e.g. Vcc/2 instead of Gnd.

  3. Scale R up and C down to minimize current
  4. Check input bias current offset voltage and balance both input R equiv with suitable R's. on (+) input.

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