As as I understand it, a higher order LPF will have a steeper roll-off curve. Something like below would happen if we increase the number of order for the LPF:
Normal LPF frequency response curve:
However, when I tried to check the frequency response curve for the 8-order LPF switched capacitor, MAX7405 datasheet page 5, it shows a less steep curve behavior which is similar to a normal second-order LPF.
8-order LPF MAX7405 frequency response curve:
For a 1k cut-off, I expect a -40dB drop at 2kHz – but the 8-order does not seem to behave like that. Does anyone know why?
Best Answer
For a lowpass with order n=8 you can expect a magnitude drop (far above the cut-off frequency) of 48dB/Octacve. I think, the presented curve does show such a slope - however, only approximately. Why do you expect a damping of 40 dB at 2 kHz?
More than that, Bessel filters are optimized with respect to their phase response (linear). The price paid for this linearized phase response is a magnitude which exhibits a relatively broad transition between passband and stopband.