1.1.1 Analog, Digital, or Mixed-Signal?



Before delving into the details of mixed-signal IC test and measurement, one might first ask a few good questions. Exactly what are mixed-signal circuits? how are they used in typical applications? Why do we have to test mixed-signal circuits in the first place? What is the role of a test engineer, and how does it differ from that of a design engineer or product engineer? Most training classes offered by mixed-signal tester companies assume that the students already know the answers to these questions. For instance, a typical automated test equipment (ATE) training class shows the students how to program the per-pin current leakage measurement instruments in the tester before the students even know why leakage current is an important parameter to measure. This book will answer many of the what’s when’s, and why’s of mixed-signal testing, as well as the usual hows. let’s start with a very basic question: what is a mixed-signal circuit?



A mixed-signal circuit can be defined as a circuit consisting of both digital and analog elements. By this definition, a comparator is one of the simplest mixed-signal circuits. It compares two analog voltages and determines if the first voltage is greater than or less than the second voltage. Its digital output changes to one of two states depending on the outcome of the comparison. In effect, a comparator is a one-bit analog-to-digital converter (ADC). It might also be argued that a simple digital inverter is a mixed-signal circuit, since its digital controls an "analog" output that swings between two fixed voltages, rising, falling, overshooting, and undershooting according to the laws of analog circuits. In fact, in certain extremely high-frequency applications, the outputs of digital circuits have been tested using mixed-signal testing methodologies.



Some mixed-signal experts might argue that a comparator and an inverter are not mixed-signal devices at all. The comparator is typically considered an analog circuit, while an inverter is considered a digital circuit. Other examples of borderline mixed-signal devices are analog switches and programmable gain amplifiers. The purist might argue that mixed-signal circuits are those that involve some sort of nontrivial interaction between digital signals and analog signals. Otherwise, the device is simply a combination of digital logic and separate analog circuitry coexisting on the same die or circuit board. The line between mixed-signal circuits and analog or digital circuits is blurry if one wants to be pedantic.




    News123 發表在 痞客邦 留言(0) 人氣()