Low-noise low-power allpole active-RC filters minimizing resistor level

The design procedure of 2^nd- and 3^rd-order low-sensitivity low-power allpole active resistance-capacitance (RC) filters, using the impedance tapering design method has already been published. Beside sensitivity minimization the impedance tapering was used for output noise minimization and output dynamic range maximization. In this paper it is shown that active resistance-capacitance (RC) filters can also be designed for low input thermal noise. The design procedure of filters with minimum input noise is very similar to the design of minimum sensitivity filters in its most important steps. Thus, the judicious selection of component values accounts for the considerable decrease in sensitivity and in input thermal noise. The minimum noise at the filters input is obtained by minimizing the total resistance of the filter circuit. The noise analysis was performed on the 2^nd- and 3^rd-order (class 4) Sallen and Key low-pass filter sections using MATLAB.