A parallel structure of a continuous-time filter for band-pass sigma-delta A/D converters

In classical continuous-time band-pass sigma-delta (ΣΔ) modulators, the loop filter transfer function is a serial structure of biquadratic functions with high gains and high quality factors. Unfortunately, such a transfer function cannot be directly designed with passive components. We propose a parallel structure consisting in resonators and a low-order filter. The first branch is a cascade of high quality factors resonators that guarantee a high gain in band. The second branch is a smooth filter that guarantees the ΣΔ loop stability out of band. Filters characteristics are obtained thanks to an optimization method to match the reference filter as much as possible in the appropriate band. Each block can be easily designed. Moreover, simulations show that sensitivity to gain variations is improved.