Analog Integrated Circuit Design, 2nd Edition

Perhaps the second most widely used electronic components (after amplifiers) are comparators. A comparator is used to detect whether a signal is greater or smaller than zero, or to compare the size of one signal to another. As we will see in Chapter 17, comparators are used in large abundance in A/D converters. They also find widespread use in many other applications, such as data transmission, switching power regulators, and others. In this chapter, we look at comparator design and practical limitations, where a number of different approaches are discussed. First, we examine a simplistic approach of using an open-loop opamp for a comparator. Although this approach is too slow for practical applications, it is a good example to use when we discuss several design principles for minimizing input-offset voltage and charge-injection errors. A number of other approaches are also described, such as multiple-stage comparators, positive-feedback track-and-latch comparators, and fully differential comparators. Finally, examples of both CMOS and bipolar comparator circuits are also described.

10.1   COMPARATOR SPECIFICATIONS

10.1.1   Input Offset and Noise

The input offset of a comparator is the input voltage at which its output changes from one logic level to the other (neglecting noise). In an ideal comparator, this value would be zero. In a practical comparator, input offset might be ...