I have here a circuit of butterworth 2d order with a cutoff frequency of 38KHz from calculation:
fc=1/(2*pi*sqrt(R223*R225*C190*C191))=38.73146646102262e+03
but in matlab, the plot of the transfer function gives me another different cut off frequency at -3dB
R223=1.6e3;
R225=3.9e3;
C191=820e-12;
C190=3.3e-9;
Anti_aliasing=tf([0 1],[(R223*R225*C190*C191) (R225*C191+R223*C191) 1]);
options = bodeoptions;
options.FreqUnits = 'Hz'; % or 'rad/second', 'rpm', etc.
figure('name','2d order butterworth filter with 38khz');
bode(Anti_aliasing,options);
fc of bode plot=47e03 Hz
I have the same value and the transfer function seems correct. what am I missing?