Returns equivalent IIR coefficients for an analog RC filter
Usage: [B,A] = RC_FILTER(r, c, fs, type);
R is the resistance value (in ohms)
C is the capacitance value (in farrads)
FS is the digital sample rate (in Hz)
type is a character string defining filter type
Choices are: 'high' or 'low'
This function uses a pre-calculated equation for both of these circuits that only requires the resistance and capacitance value to get a true digital filter equivalent to a basic analog filter.
The math behind these equations is based off the basic bilinear transform technique that can be found in many DSP textbooks. The reference paper for this function was "Conversion of Analog to Digital Transfer Functions" by C. Sidney Burrus, page 6.
This is also the equivalent of a 1st order butterworth with a cuttoff frequency of Fc = 1/(2*pi*R*C);
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