Signal Processing and Integrated Circuits

Chapter 16

Non-ideal Effects and Practical Considerations in Microelectronic Switched-capacitor Filters

16.1 Introduction

In the previous chapter, the design techniques of switched-capacitor filters were delineated assuming idealized components. Further, Chapters 10–14 dealt with the integrated circuit building blocks together with their non-ideal behaviour. In this chapter, we combine the results to examine the effect of the non-ideal behaviour of the integrated circuit building blocks on the overall response of switched-capacitor filters [24, 27]. We also examine some practical issues, which may be of interest to the designer. Many of these effects are also applicable to analog continuous filters.

16.2 Effect of Finite Op Amp Gain

The building blocks of switched-capacitor filters are first or second order sections containing Op Amps, switches and capacitors. In the design methods presented in the previous chapters, it has been assumed that the Op Amps are ideal, having infinite gain values. However, as we know from Chapters 12–13, real CMOS Op Amps have large but finite gain values. This factor has to be taken into account in the final simulation of the switched-capacitor filter to determine its exact response, particularly for high-frequency designs where the operating frequencies approach the bandwidth of the Op Amp. The finite gains of the Op Amps result in distortion of the transfer functions of the building blocks and hence the overall filter transfer function. Furthermore, ...