A Design of a Programmable Impedance Scaling Circuit for Low Frequency Applications

Abstract

A large capacitor is needed for low-frequency active filters to process biomedical signals. The large capacitor is implemented by an impedance scaling circuit. However, the apparent capacitance of the conventional impedance scaling circuit cannot be tuned because that the scaling factor of the impedance scaling circuit is designed by the size of transistors of a negative current feedback circuit. This paper presents a digitally programmable impedance scaling circuit using electrical switches. The proposed circuit is realized by controlling the number of parallel MOS devices which compose the negative current feedback circuit using electrical switches. The validity of the proposed method is confirmed by simulations using HSPICE. The frequency characteristics of the proposed impedance scaling circuits using ideal switches are not deteriorated as compared with one of the conventional impedance scaling circuit. If the capacitor varies from −22% to +49.5%, the tuning error of the proposed impedance scaling circuit is within 0.5%. The proposed circuit can be applied to a 2nd order Butterworth filter whose cutoff frequency is 100 Hz.