Decimation filters for Sigma - Delta A/Ds

A typical filter used as the pre-decimation filter for an oversampled A/D is the Hogenauer filter, also called the CIC filter. These filters have some advantages which make them particularly suitable for use as decimation filters. In general the output stream from a OSR A/D is a 1 bit high frequency digital signal. The 1 bit signal has to be downconverted in frequency and increased in bit width. This is the fundamental decimation operation. Hogenauer filters offer the following advantages (1) No multipliers are needed. (2) No storage is needed for filter coefficients. (3)Intermediate storage is reduced by integrating at the high sampling rate and comb filtering at a low rate. (4) The structure of the CIC filters is very uniform, using only two basic building blocks. (5) Little external control or complicated local timing is required. (6) The same design can easily be used for a wide range of rate change factors with the addition of a scaling unit. As a result of these advantages Hogenauer filters have been used and continue to be used in overampled systems. A technical report prepared by technical staff at Signal Processing Group Inc. is now available in a series of posts that deal with the Hogenauer filter as well as OSR A/D converters. It was assumed that since the CIC filter is an important component at the backend of an OSR ADC, understanding the design parameters of this filter is essential to the design of the overall OSR ADC. Subsequent posts to this one deal with the details of design for decimation filters. The paper may be found at http://www.signalpro.biz>engineer's corner.