4.1 Introduction

Log domain filters are amongst the newest of ideas in the well developed field of electronic filter design. These filters offer reduced circuit complexity, wider bandwidth, wider dynamic range and lower power consumption in certain applications than their counterparts. For example, with typical signal swings of 100 mV, it is possible to achieve 60 80 dB of dynamic range in circuits operating in the MHz range using power supplies of 2.7 V or less. Moreover, these filters are widely tunable electronically, making them a very interesting choice in such applications as magnetic read channels. We will explore these possibilities and the limitations of log domain filters in this chapter, along with ways of designing and understanding these filters. Beyond the possible performance improvements which may be achievable, they offer a view into an entirely new way of looking at linear filters. Specifically, log domain filters are a type of externally linear, internally nonlinear (ELIN) filters, which we may call a special case of externally linear filters, which includes all circuits that provide a linear transfer function from the input to the output. This broader perspective has not been used historically because it has been tacitly assumed that input-output linear filters had to be made with linear components, or at least linearised components.

Linear filtering is a concept that has existed since the earliest stages of electronic design. Classically the 'linear' part of this terminology is a natural consequence of the linearity of the equations used to derive...