A Noise-Shaping Algorithm of Multi-bit DAC Nonlinearities in Complex Bandpass ∆ΣAD Modulators

This paper presents a technique for improving the SNR and resolution of complex bandpass ∆ΣADCs 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 ∆ΣAD 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 ∆ΣADC degrades. For the conversion of complex intermediate frequency (IF) input signals, a complex bandpass ∆ΣAD modulator can provide superior performance to a pair of real bandpass ∆ΣAD 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 ∆ΣAD 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 ∆ΣADC with nonlinear internal multi-bit DACs. key words: complex bandpass ∆ΣAD modulator, bandpass filter, noiseshaping, ADC, DAC, element rotation, data-weighted averaging, low-IF receiver