chapter  9
70 Pages

Digital Interfaces in Measurement Systems

As we have seen in Figure 1.1, a modern instrumentation system generally includes a computer which is used to supervise, coordinate and control the measurements, and is often used to store data (data logging), condition data display it in a meaningful, summary form on a monitor. In this chapter, we describe the hardware associated with the conversion of analog information to digital formats, and noise and resolution problems associated with the analog-to-digital conversion (ADC) process. We also examine various means of digital communication with instruments, including

the IEEE-488.2 bus, various serial data protocols and other parallel bus architectures used with PCs. Commercially available interface cards are described and problems associated with sending both analog and digital data over long distances at high data rates on cables are analyzed. In describing data conversion interfaces, it is expedient to first consider digital-to-analog

converters (DACs) because these systems are used in several designs for analog-to-digital converters (ADCs). Data conversion from analog to digital or digital to analog forms is often done continuously and periodically. Continuous, periodic data conversion has a significant effect on the information content of the converted data, as we will show in the next section. Theoretically, a noise free analog signal sample has infinite resolution. Once the analog signal sample has been converted to digital form, it is represented by a digital (binary) number of a finite number of bits (e.g. 12), which limits the resolution of the sample (one part in 4096 for 12 bits). This obligatory rounding off of the digital sample is called quantization and the resulting errors can be thought of as being caused by a quantization noise. It is shown in Section 9.4 that adding broadband noise (dithering) to the analog signal being digitized mitigates the effect of quantization noise on signal resolution. Integrated circuit ADCs and DACs are available in a wide spectrum of specifications.

They are available with quantization levels ranging from 6 to over 20 bits and with conversion rates ranging from low audio frequencies to less than 1 ns per sample. Most ADCs and DACs have parallel digital outputs or inputs, respectively, although a few are designed to use serial data I/O protocols. Before discussing data conversion hardware, we will first discuss the linear dynamics

of data conversion, quantization noise and dithering.