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                                         THE SYSTEMS APPROACH TO CONTROL AND INSTRUMENTATION               2




                                              The circuit that converts sampled analog data to binary values is called an
                                         analog-to-digital converter (ADC). It is also sometimes called a quantizer.  When
                                         the computer has finished calculations on a given sample, it outputs a
                                         numerical value to an external device in the form of a binary number. If that
                                         device is analog, as in the case of an actuator for a control system, the output
                                         binary number must be reconverted to analog format. This conversion is done
                                         in a device known as a digital-to-analog converter (DAC). The two types of
                                         converters are explained in detail in Chapter 4. Not all digital electronic
                                         systems require converters, because some sensors and actuators are digital
                                         already. Except for these cases, a digital electronic system for either control or
                                         instrumentation has a block diagram as depicted in Figure 2.5.


                    Figure 2.5
                    Digital Control (a) or Measurement (b) System Configuration























                                         ELECTRONIC SYSTEM PERFORMANCE
                                              The performance of an electronic system is evaluated by quantitative
                                         descriptions of how well it performs its intended task and inherently uses
                                         numerical representation (e.g., parameters and graphs). The home audio
                                         system, which has been mentioned previously as a familiar example of an
                                         electronic system, will serve as an example for performance analysis.
                                              The performance of a high-fidelity home audio system is described by
                                         such characteristics as frequency response, maximum power level, harmonic
                                         distortion, and linearity (as well as other characterizations that are specific to an
                                         audio system). The “fidelity” of an audio system, which expresses how well it
                                         reproduces the sounds from the source, is best given by its frequency response.
                                         Unwanted distortion of the sound is characterized by harmonic distortion and
                                         linearity. Similar measures are also appropriate and useful for characterizing an
                                         electronic system.

                                         UNDERSTANDING AUTOMOTIVE ELECTRONICS                             37
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