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2nd Order Lowpass Filter

Schematic of a second order lowpass active filter.

    Entering a value for the cutoff frequency (-3db or 0.707 point) will find the optimum resistor and capacitor values for the highpass filter shown. You can optionally enter a value for R or C along with the cutoff frequency to find the missing value. Entering a value for both R & C finds the frequency.

    By default the solver finds values for a Bessel implementation. For general information about filters and specific details about filter implementations, see the Filter Types article. You can change the implementation by entering a value for Type. The following table lists the acceptable values for Type and the filter implementation for each Type value.

Table describing the filter type options.

    Unless otherwise specified, the capacitor’s tolerance is assumed to be 5%. If you wish to specify a different tolerance, enter it as a percentage (10% = 10).

    Even though solutions are provided for a number tolerance bands, this filter design (Sallen-Key or VCVS) performs best when the component values are more closely matched. The use of 1% tolerance resistors and reasonably high grade film capacitors are recommended. In addition, it is best to use resistor values that are greater than 10K (other than Rg) to minimize frequency errors.

    If you are interested in the details, an excellent downloadable resource is available an Application Report from Texas Instruments. Search for SLOA049B, “Active Low-Pass Filter Design” by Jim Karki. Excellent, and concise, coverage of the subject matter is also given in the Art of Electronics by Horowitz and Hill.

copyright © 2021 John Miskimins


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