20 - Analogue-digital data conversion

Abstract

This chapter covers the circuits that interface between analog and digital electronic circuitry. The Digital-To-Analog Converter (DAC) gives the analog output Vo proportional to the normally parallel digital input Din. Sometimes the operation is unipolar, with Din unsigned, where the codes are interpreted as being always positive, with Vo positive in proportion. Otherwise the operation is bipolar, with Din signed (ranging from negative to positive) with Vo again in proportion. With the modifications described, the unsigned DAC can be converted to signed operation, sometimes requiring two of the unsigned devices. The Multiplying DAC (MDAC) has one analog input Vin and one digital Din, with Vo μo,Vin x Din. An MDAC with Din unsigned and Vin only positive is said to be “single quadrant.” This becomes two quadrants for either Din signed or Vin bipolar, and four quadrants for both bipolar. Methods of conversion from single- to two-quadrant operation are described. One of the limitations of a DAC is a brief, but often quite large transient, or glitch that occurs at the point when Din changes from one value to the next. The associated settling time and methods of avoiding the glitch are explained in the chapter. For the ADC, the digital output DO is proportional to the analog input Vin. The ADC too can be bipolar or unipolar. The types of DAC described in the chapter are the switched-output and switched-input ladder DACs, the higher speed 0800 type, and the oversampling DAC. The ADC types are the successive approximation, flash, dual ramp, and oversampling types.