# Second order active low pass filter values

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

• Are you sure this schematic you have given will work as a Second order low pass filter ? Dec 29, 2017 at 15:34
• @MITURAJ that is actually a classic second-order Sallen-Key LPF Dec 29, 2017 at 16:05
• @MITURAJ oh this will help a lot with the calculations thank you
– user170589
Dec 29, 2017 at 16:10
• @MITURAJ great ref. used it too many times ! Dec 29, 2017 at 16:10
• No linear filter will convert a 33 kHz input signal to a 30 kHz output signal. Dec 29, 2017 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.