Nyquist-rate digital-to-analog converter architectures - IEEE Potentials

he ever-dropping cost of Very Large Scale Integrated (VLSI) circuits allows many analog functions to be done digitally. However, the real world still is, and will always continue to be, a fundamentally analog place. Thus, the analog signal of interest is translated into a format that a digital computer can utilize. This translation is the function of an Analog to Digital Converter (ADC). After processing, the resulting digital stream of information is returned to an analog form by a Digital to Analog Converter (DAC). An analog-signal once again, the information can be consumed by human senses or manipulated by analog circuits. Figure 1 shows the information conversion cycle between the analog and digital domains. ADCs and DACs are ubiquitous in computing systems. Many electronic products, including compact disc players, camcorders, digital cellular phones, modems, computer sound cards, computer graphics adapters, and high definition televisions, contain one or more data converters. For many purposes, a sufficient interpretation is that the converter accepts an analog input, typically a voltage or current. It then provides an n bit digital output which represents the ADC’s input. ADCs are implemented with many, and widely varying, architectures. Several sources, listed at the conclusion of this article, can provide a glimpse into the diverse world of ADCs. Viewed as a black box, the DAC has an n bit digital word input. Its analog output is proportional to the DAC’s input. The DAC’s output is typically a voltage or current. A wide variety of circuits have been designed to perform digital to analog conversion. The astounding assortment of data converter architectures will quickly overwhelm the uninitiated. An understanding of how conversions are done will allow the engineer to make the proper choice for a given application. DACs can be classified into two categories, Nyquist-rate DACs or oversampling DACs, according to their operation. In general, Nyquistrate DACs operate on data samples generated their outputs at the sampling frequency. Nyquist-rate DACs typically have circuits that very obviously convert a digital number into an analog quantity. Oversampling DACs operate on data samples generated at frequencies that are much higher than the sampling frequency. Oversampling DACS usually have relatively simple circuitry, but use complex signal processing techniques to complete the data conversion. An excellent reference is provided at the conclusion of this article. This article covers two popular types of Nyquist-rate DACs: the flash DAC and the serial DAC. Flash DACs perform their conversion in a single clock cycle and are typically designed to operate at high speeds. Serial DACs convert the digital signal to an analog signal one bit at a time. Serial DACs trade the hardware complexity of a flash DAC for longer conversion times. In this article, three variations of flash DACs are introduced along with two serial DACs. Also, the advantages and disadvantages for each architecture will be discussed.