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 how's. 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...