How can i decide what filter approximation to use ... Chebyshev, Butterworth or Bessel etc?
The selection of a filter approximation can be determined by considering a frequency and time response trade-off. A Chebyshev LPF amplitude response has higher stopband attenuation to Butterworth or Bessel LPF of equal order. For a Chebyshev LPF, the stopband attenuation increases with increasing passband ripple. A Bessel LPF transient response has a lower overshoot and shorter settling time than a Butterworth or a Chebyshev LPF of equal order. Typically, a filter approximation is selected for signal chain requirements. If the input signal is a pulse or a square wave, then a Bessel LPF minimizes waveform distortion. If the input signal slew rate is low, then a Butterworth or Chebyshev LPF maximizes stopband attenuation. The frequency and time response plots of fourth order LPF show the response of four LPF approximations:
1dB Ripple Chebyshev, 0.1dB Ripple Chebyshev, Butterworth, and Bessel.