Sallen Key lowpass filter calculator, Boode plot

oct. 2015, Jeroen Korterik

The circuit

The Sallen Key implementation of a lowpass filter is shown below:
2nd order Sallen Key lowpass filter

Calculator

R1 value:
 [Ohms]
Cutoff frequency:
 [Hz]
R2 value:
 [Ohms]
C1 value:
 [Farad]
C2 value:
 [Farad]
Actual Cutoff Frequency:
 [Hz]
Q value:

Boode plot of the transfer function

The gain and fase vs. frequency response of the lowpass filter is shown below. Refer to the derivation of the response for more info (doc file).

How to use this tool

This webpage calculates the transfer of a 2nd order Sallen Key lowpass filter. Enter a value for R1 and the cutoff frequency and click calculate the values for R2, C1 and C2 in case of a Butterworth or Bessel response.
You can as well enter R1, C1 and C2 manually and check the response.
Results: cutoff frequency and q factor, Boode plot of the transfer function.

Yet another filter calctool, why??

There are a few reasons:
  • it shows the Boode plot (gain and phase vs. frequency)
  • to demonstrate log-log and lin-log plots on the HTML5 canvas
  • just to play a bit with javascript and HTML5 canvas :)