LECTURE NOTES ADCs AND DACs
By: Olorunniwo O. Dept. of Electronic & Electrical Engineering Obafemi Awolowo University Ile-Ife Page. | 1
ADCs and DACs • Basic DAC Architecture: -binary weighted; -R-2R ladder • Basic ADC: -flash; -dual-slope ; and -successive approximation • Sample and Hold Circuit • Aperture Error • ADC/DAC specifications: -INL; -DNL; -dynamic range. • Oversampling • Noise-shaping and sigma-delta converters 1.1 INTRODUCTION
It is observed that most physical quantities in nature are analog signals. These quantities include: temperature, pressure and speed—to mention a few. Further, these quantities assume an infinite number of possible values other than 1’s and 0’s. Understandably, these analog quantities must be converted into binary strings, representative of their values. The process of conversion of analog signals into their equivalent digital form is termed analog-to-digital conversion. The reverse form of the process is digital-to-analog conversion. Often, computers utilize these important processes for control of analog devices. To that end, for analog-to-digital (A/D) and digital-to-analog (D/A) converters to be useful, there has to be a meaningful representation of the analog and signal quantity. Starting points of the conversion processes are with devices called transducers. Transducers are devices that convert physical quantities into electrical quantities. What are the major building blocks in ADC and DAC circuits? The operational amplifiers are common features in A/D and D/A converters. It provides means of: 1. Summing currents at the input; and 2. Converting a current to voltage at the output of the converter circuit. The basic operations of an ideal op-amp are illustrated in Figure 1 and 2.
Figure1 Page. | 2
Figure2 Note: The three (3) notable characteristics of an op-amp are: (a) Very high input impedance; (b) Very high voltage gain; and (c) Very low output impedance. These characteristics are necessary requirements of an op-amp used for signal conditioning. Various configuration using op-amps includes: comparator, adder/summer, buffer circuits, inverting and noninverting amplifiers, etc. 1.2 1.2.1 Basic DAC Architecture Binary-weighted Digital-to-Analog Converter
The aim of a DAC is to convert analog physical quantities into discretized outputs levels. Figure 3 illustrates a binary weighted D/A converter. Using various input resistors as binary weighting factors, each input can be made to provide a binary weighted amount of current. Additionally, the output voltage will represent a sum of all binary-weights of the input currents. Table 1 highlights several switch combinations and the corresponding output levels. Table 1 D4 0 0 0 0 0 0 0 0 : Figure3 Each successive switch makes twice the amount of the previous switch. Thus, the DAC in Figure 3 is for a 4-bit conversion. Moreover, an 8-bit D/A converter would require 8 successive resistors half the values of the previous one. If D8 switch correspond to 100k , then switch D1 must correspond to 0.78125k . Of course, a precision resistor would suffice for this requirement, though accurate resistances over such a large range of values are very difficult to achieve. Therefore, this limits the practical use of most D/A converters. D3 0 0 0 0 1 1 1 1 : D2 0 0 1 1 0 0 1 1 : D1 0 1 0 1 0 1 0 1 : Vout (-V) 0 1 2 3 4 5 6 7 :
Page. | 3
R-2R Ladder Digital-to-Analog Converters
A commonly used D/A conversion method in integrated D/A converters circuits (IC) known as R/2R ladder. An R-2R ladder D/A converter is shown in Figure 4.
Figure4 For this circuit, only two resistance values are required, that is R and 2R, which lends itself to ease of fabrications of R-2R ladder ICs with increasing resolutions of 8, 10, or 12bits, and higher. Furthermore, to improve on the resolution of R/2R converter, simply, additional R/2R resistors are implemented with corresponding...