7.1 Introduction

We have learnt from the material in the previous chapters (refer also to the references in Reference 1) that a variety of well understood methods is available for designing continuous-time, or analogue, filters in fully integrated form for operating frequencies ranging from the audio range to RF. The synthesis strategies were seen to be the same, in principle, as the ones employed for any analogue active filter; they make use of capacitors, resistors and gain, that is, active devices, in a feedback configuration. Integrated filters can be implemented in any integrated-circuit (IC) technology, such as CMOS, bipolar or GaAs. The preferred technology is CMOS because analogue filters will normally reside on predominantly digital ICs and for economic reasons must be compatible with digital IC technology.

As all analogue filters, integrated filters require accurate component values to realise the prescribed frequency, quality factor and gain parameters. These are normally not available in an IC because of process tolerances and varying operating conditions, and changing the components on an IC may prove difficult or impossible. To provide more room for accommodating parameter deviations, the designer may be tempted to solve tolerance problems by designing a filter with tighter passband and steeper roll-off constraints than required. This solution can as a rule not be recommended because it leads to a higher-order filter, uses more power and area, and increases costs. In addition, the higher filter order and steeper roll-off characteristics result in larger sensitivities [2],...