Design of an on-line multiply-add module for recursive digital filters

An online multiply-add module that allows high filter sampling rates when used to implement the direct form II second-order filter structure is described. Important characteristics of online arithmetic are that it produces most significant digit first and that its digit cycle time is independent of the data wordlength. These features not only permit high-speed filtering, but also allow the elimination of all nonlinear oscillation in the filter without affecting the sampling rate, and effectively eliminate scaling of the filter´s input data. The derivation of the online multiply-add algorithm and its hardware design using a 1.5-μm CMOS standard cell library are presented. A method for eliminating nonlinear oscillations by increasing the filter´s working precision is described