Determining appropriate precisions for signals in fixed-point IIR filters

This paper presents an analytical framework for the implementation of digital infinite impulse response filters in fixed-point hardware on field programmable gate arrays. This analysis is necessary because FPGAs, unlike fixed register size digital signal processors, allow custom bit widths. Within the framework, the designer determines the number of bits necessary for representing the constant coefficients and the internal signals in the filter. The coefficient bit widths are determined by accounting for the sensitivity of the filter´s pole and zero locations with respect to the coefficient perturbations. The internal signal bit widths are determined by calculating theoretical bounds on the ranges of the signals, and on the errors introduced by truncation in the fixed-point hardware. The bounds tell how many bits are required at any point in the computation in order to avoid overflow and guarantee a prescribed degree of accuracy in the filter output. The bounds form the basis for a methodology for the fixed-point digital filter implementation. The methodology is applied to the implementation of a second-order filter used as a compensator in a magnetic bearing control system.