Simplified rotated sinc (RS) filter for sigma-delta A/D converters

This paper presents a simplified rotated sinc (RS) filter where the zero-rotation is introduced only in the first folding band. As a result, good high alias rejection is achieved in the first folding band, where typically the worst case aliasing occurs. In contrast to the original RS filter design, the simplified RS filter does not have the problem of pole-zero cancellation for the quantized coefficients in a recursive implementation. Additionally, using the polyphase decomposition, the zero-rotation term (ZRT) is moved to a lower rate. The coefficient in the ZRT can be expressed as a sum of powers of two, thereby avoiding multiplication operation altogether. Additionally, attenuation in the folding bands is improved by a cascade of expanded cosine filters, which can also be moved to a lower rate. As a consequence, there is no filtering at the high input rate in the overall structure, and the structure is a multiplier-free.