Introduction to CMOS OP-AMPs and Comparators

CHAPTER 6

DIGITAL-TO-ANALOG CONVERTERS

The analog-to-digital (A/D) and digital-to-analog (D/A) converters are the main link between the analog signals and the digital world of signal processing. Data converters are generally divided into the two broad categories of Nyquist-rate and oversampling converters. Nyquist-rate converters are converters that operate at 1.5 to 5 times the Nyquist rate (i.e., a sample rate of 3 to 10 times the signal's bandwidth), and each input signal is uniquely represented by an output signal. Conversely, oversampling or delta-sigma converters operate at sampling rates that are much higher than the input signal's Nyquist rate and increase the output signal-to-noise ratio by subsequent filtering that removes the out-of-band quantization noise. The ratio of the sampling rate to the Nyquist rate is called the oversampling ratio. For most practical delta-sigma converters, the oversampling ratio is typically between 16 and 256.

A Nyquist-rate digital-to-analog converter (DAC) is a device that converts a digital input signal (or code) to an analog output voltage (or current) that is proportional to the digital signal. In this and the following chapters a variety of methods are presented for realizing Nyquist-rate converters. Oversampling converters are not discussed in this book; the interested reader is referred to Ref. 20 for an in-depth coverage of this subject. The chapter begins with a general introduction and characterization of the converters, followed ...