Anti-aliasing filter improvement in continuous-time feedback sigma-delta modulators

This paper describes a fundamental limitation of the implicit anti-aliasing filter in continuous-time feedback compensated sigma-delta modulators and a method to overcome it. This anti-aliasing filter can be used to relax an additionally required pre filtering stage. If a strong alias rejection is required, commonly used continuous-time feedback compensated sigma-delta modulators would need some pre-filtering or a large oversampling ratio. The described technique generates a notch at the sampling frequency to improve the performance of the antialiasing filter. This method is possible without additional active components or large oversampling ratios. It is also very robust against component mismatch.