Low-noise active-RC low-, high- and band-pass allpole filters using impedance tapering

It is shown that low-sensitivity active resistance-capacitance (RC) filters are also low in output thermal noise. The design procedure of second- and third-order low-sensitivity allpole filters, using impedance tapering, has already been published. The component values selected for impedance tapering account for the considerable decrease in output thermal noise. The method of Zurada and Bialko (1975) was used to determine output noise spectral density and total RMS output noise of filters. Passive elements and operational amplifiers are represented by substitute noise models. The noise contribution of each device to the output node is calculated using noise transfer functions. The noise analysis was performed on the second-order (class 4) Sallen and Key low-pass, high-pass and band-pass filter sections using MATLAB. The extension to third- and higher-order filters follows the same principles. It was found that the filters with minimum noise coincide with the filters with minimum sensitivity to component tolerances.