Data Acquisition

Figure 3.9Ideal band stop filter transfer characteristicsThe transfer characteristics of a practical filter for minimum ‘Q’ and maximum ‘Q’ areshown in Figure 3.10.Figure 3.10Practical active notch transfer characteristicsButterworth filterButterworth filters provide a higher level of low pass filtering, containing two or morelow pass filter stages. The number of stages ‘n’ of the filter determines how sharp theroll-off is at the cut-off frequency. A two-stage filter of this type is known as a secondorder Butterworth filter as shown in Figure 3.11.A fourth order Butterworth filter would have two of the filter sections shown in Figure3.11 cascaded together.