An improved technique for reducing baseband tones in sigma-delta modulators employing data weighted averaging algorithm without adding dither

The data weighted averaging (DWA) algorithm used in multibit sigma-delta modulators (SDM) is troubled by baseband tones resulting from component mismatch of the SDM's internal multi-bit digital-to-analog converter (DAC). In this paper, we analyze DAC baseband tones and find them closely correlated to the number of unit elements used in the DAC. An improved technique is proposed for shifting the DAC tones away from the baseband without adding dither, For third-order sigma-delta modulators with an oversampling ratio (OSR) of 64 and either a nine-level or eight-level internal DAC with 0.5%-2% random component mismatches, simulation reveals that the DWA algorithm with the proposed technique can achieve nearly perfect first-order DAC noise shaping in the baseband and, on the average, 12 dB improved signal-to-(noise and distortion) ratio and 20 dB improved in-band distortion.