Finite word length effect in IIR filters

ByM.NITHIYANANDHAMV.SHANKAR RAM

INTRODUCTION

• The coefficients obtained from 2 stage FIR filter is of infinite length

• These degrade the performance of filter• These errors can be reduced by using more bits• The effects of finite word length on performance are

more difficult to analyze in IIR filters than in FIR filters because of feedback arrangement

ErrorsADC Quantization noise

Coefficient Quantization errors

Overflow errors

Product round off errors

ErrorsDegrade the performance of filter

Filter leads to instability

Filter Degradation Depends onWord length and type of arithmetic used to perform the

filtering operation

Method used to perform quantize filter coefficient and variable

Filter structure

Influence of filter effectsDirect form 1

Direct form 2

Direct form 1

Direct form 2

Cascade Realization

Parallel Realization

Coefficient Quantization error in IIR filters

Additional overflow errorsIt is due to addition of two large numbers of

similar sign.Because of recursive nature of iir filter , an

overflow at w(n) is fed back and used to compute the next output where it cause an further overflow.