I am a student trying to design a Butterworth LPF to meet the class 1000 specifications, by SAE J211 (https://law.resource.org/pub/us/cfr/ibr/005/sae.j211-1.1995.pdf). My sensor is an ADXL377 accelerometer with analog output. My filter requirements (from J211) are fp = 1000Hz (Pass band frequency) fc = 1650Hz (Cutoff frequency) Rp = 0.5db (Pass band ripple) Rs = -40db (Stop band ripple) Fs = 10,000Hz (Sample rate) My goal is to come up with some filter design that I can then implement with an RC circuit. J211 specifies that it a 4th order butterworth could be used to meet the above requirements but when I try inputting the values on Matlab I obtain an unstable filter. Wp = 2*fp/Fs fs comes from 4th order filter with -24octave/db, and fc at -3db. fs = 3590Hz Ws = 2*fs/Fs [n,Wn] = buttord(Wp,Ws,Rp,Rs); [b,a] = butter(n,Wn); G = tf(b,a); isstable(G) —> outputs zero Also the poles are in the right side of the imaginary axis in the pzplot... Should I just use the “designfilt” function and take whatever it gives me? Could someone help me out?