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.(adsbygoogle = window.adsbygoogle || []).push({});

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?

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# Homework Help: Filter Design -- Butterworth LPF for aan ADXL377 accelerometer with analog output

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