Active Low-Pass Filters The 3-dB frequency c is related to o by a factor known as the FREQUENCY CORRECTION FACTOR (kLP), thus; Parameter table for Sellen-Key 2nd order LPF Type of response  kLP Bessel 1.732 0.785 Butterworth 1.414 1 Chebyshev (1 dB) 1.054 1.238 Chebyshev (2 dB) 0.886 1.333 Chebyshev (3 dB) 0.766 1.390

Example Design a Bessel 2nd order low-pass filter with a 3-dB frequency of 5 kHz. Use C = 22 nF.

Solution From the table: Hence:

Solution In order to minimize the offset error, we have to meet the following condition: Or; And: From the parameter table;

Solution

Solution A Bessel 2nd order LPF

Frequency response curve of a 2nd order Bessel LPF Solution fc Frequency response curve of a 2nd order Bessel LPF