A Polyphase Filter Design for Continuous-Time Quadrature Bandpass Sigma–Delta Modulators

This paper presents a strategy for successful polyphase-filter design for continuous-time quadrature bandpass sigma-delta (SigmaDelta) modulators. Based on a low-pass filter with a chain of integrators with weighted capacitive feedforward summation (CICFF) topology - which is suited for implementation in low-power applications - analytical equations are derived. A new compensation scheme is proposed and implemented by cross-coupling additional resistors, without the necessity of extra-active components. Translation to intermediate frequency in second- and fourth-order polyphase filters with the proposed compensation scheme are compared to analytical considerations and simulation. Nonlinearities introduced by mismatch of feedforward coefficients and finite gain-bandwidth of amplifiers are considered.