Polyphase Implementation of Non-recursive Comb Decimators for Sigma-Delta A/D Converters

In a sigma-delta analog to digital (A/D) converter, the most computationally intensive block is the decimation filter and its hardware implementation may require millions of transistors. Since these converters are now targeted for a portable application, a hardware efficient design is an implicit requirement. In this effect, this paper presents a computationally efficient polyphase implementation of non-recursive cascaded integrator comb (CIC) decimators for sigma-delta converters (SDCs). The SDCs are operating at high oversampling frequencies and hence require large sampling rate conversions. The filtering and rate reduction are performed in several stages to reduce hardware complexity and power dissipation. The CIC filters are widely adopted as the first stage of decimation due to its multiplier free structure. In this research, the performance of polyphase structure is compared with the CICs using recursive and non-recursive algorithms in terms of power, speed and area. This polyphase implementation offers high speed operation and low power consumption. The polyphase implementation of 4th order CIC filter with a decimation factor of '64' and input word length of '4-bits' offers about 70% and 37% of power saving compared to the corresponding recursive and non-recursive implementations respectively. The same polyphase CIC filter can operate about 7 times faster than the recursive and about 3.7 times faster than the non-recursive CIC filters.