A noise-shaping algorithm of multi-bit DAC nonlinearities in complex bandpass ΔΣAD modulators

This paper presents a technique for improving the SNR and resolution of complex bandpass DeltaSigmaADCs which are used for wireless communication systems such as cellular phone, wireless LAN and Bluetooth. Oversampling and noise-shaping are used to achieve high accuracy of a DeltaSigmaAD modulator. However when a multi-bit internal DAC is used inside a modulator, nonlinearities of the DAC are not noise-shaped and the SNR of the DeltaSigmaADC degrades. For the conversion of complex intermediate frequency (IF) input signals, a complex bandpass DeltaSigmaAD modulator can provide superior performance to a pair of real bandpass DeltaSigmaAD modulators of the same order. This paper proposes a new noise-shaping algorithm-implemented by adding simple digital circuitry-to reduce the effects of nonlinearities in multi-bit DACs of complex bandpass DeltaSigmaAD modulators. We have performed simulation with MATLAB to verify the effectiveness of the algorithm, and the results show that the proposed algorithm can improve the SNR of a complex bandpass DeltaSigmaADC with nonlinear internal multi-bit DACs.