5 Signal Conditioning and Preprocessing

The topics covered in this chapter establish the connection between gas sensors and pattern recognition, the two fundamental modules of an odor-sensing instrument that are covered in Chapters 4 and 6, respectively. A number of electronic circuits are involved in integrating pattern analysis algorithms with the underlying chemical transductionmechanisms, as shown in Fig. 5.1. First, the response of the odor sensors (e.g., a resistance change) needs to be measured and converted into an electrical signal (e.g., a voltage). This operation is performed by means of interface circuits. Second, the electrical signal undergoes analog conditioning (e.g., filtering) to enhance its information content. Third, the analog signal is sampled, digitized and stored in computer memory (not covered in this chapter due to space constraints). Finally, the sampled signal is digitally preprocessed (e.g., autoscaling) in order to make it suitable for pattern analysis. This chapter is organized in three basic parts: interface circuits, signal conditioning, and preprocessing. Section 5.2 presents the fundamental interface circuits for the three primary odor sensor types: resistive, piezoelectric, and field-effect. Section 5.3 reviews the primary functions performed by analog signal conditioning circuits. Section 5.4 covers data preprocessing – the first stage of digital signal processing. The issue of sensor and instrumentation noise, one of the most important factors determining electronic-nose performance, is also reviewed in Section 5.5. The chapter con-