Dynamic range optimization and noise reduction by low-sensitivity, fourth-order, band-pass filters using coupled general-purpose Biquads

摘要

In this paper we compare two realisations of fourth-order band-pass filter sections: first is realized as a coupled structure, having negative feedback around two Biquadratic sections in cascade (CO), second is a common design of two Biquadratic sections in cascade (CA). Dynamic range optimization by equating opamp output voltage levels using signal-flow graph is presented for a single second-order GP-Biquad and for a fourth-order coupled Biquads. The reduction of sensitivity and noise is already well-known for the case of CO BP filter, with identical Biquads, when compared to the CA design. In this paper, it is shown that equating opamp output voltage levels reduces output noise even more. In that way, dynamic range is improved substantially. Because we use a general-purpose (GP) Biquad using two integrators, we can use its simple tuning features and simple design; design equations are given. Sensitivity and output thermal noise are simulated using circuit analysis program Pspice.