Butterworth Filters have one of the flattest passbands and moderate group delay. A standard Butterworth filter's passband attenuation is: -3.01dB. However, FilterSolutions allows the option of selecting any passband attenuation, in dB, that defines the filter’s cut-off frequency.
FilterSolutions also gives the designer the option of placing user-defined zeros in the stopband. A filter with stopband zeros is no longer a true Butterworth filter, but is still in the maximally flat filter family. Below are examples of Butterworth low-pass, high-pass, bandpass and bandstop filters, and the low-pass step response: