Page 217 - DCAP108_DIGITAL_CIRCUITS_AND_LOGIC_DESIGNS

Digital Circuits and Logic Design

Notes • Discuss dual-slope A/D conversion
• Describe the single-slope A/D converter
• Discuss the dual-slope A/D converter
• Describe the successive approximation A/D converter
• Describe the flash converters

Introduction

Digital-to-analog (D/A) and Analog-to-digital (A/D) conversion form two very important aspects
of digital data processing. Digital-to-analog conversion involves translation of digital information
into equivalent analog information. As an example, the output of a digital system might be
changed to analog form for the purpose of driving a pen recorder. Similarly, an analog signal
might be required for the servomotors which drive the cursor arms of a plotter. In this respect, a
D/A converter is sometimes considered a decoding device.

The process of changing an analog signal to an equivalent digital signal is accomplished by the
use of an A/D converter. For example, an A/D converter is used to change the analog output
signals from transducers (measuring temperature, pressure, vibration, etc.) into equivalent digital
signals. These signals would then be in a form suitable for entry into a digital system. An A/D
converter is often referred to as an encoding device since it is used to encode signals for entry
into a digital system.
Digital-to-analog conversion is a straightforward process and is considerably easier than A/D
conversion. In fact, a D/A converter are usually an integral part of any A/D converter.
Principle of analog-to-digital conversion

The process of analog-to-digital conversion always involves comparing two analogue signals:
an input signal and some reference signal. The comparison is carried out with a circuit called a
comparator as shown in circuit.
A comparator circuit is essentially a high-gain differential amplifier. When V > V the output
in ref
of the comparator swings to the positive supply rail, and so the output is “1”. On the other hand,
when V < V the output voltage swings to the earth rail, “0”. Thus, the comparator gives a clear
in ref
indication of which of two voltages is the larger.

12.1 Variables-Resistor Networks

The basic problem in converting a digital signal into an equivalent analog signal change the n
digital voltage levels into one equivalent analog voltage. This can be easily accomplished by
designing a resistive network that will change each digital level into an equivalent binary weighted
voltage (or current).
12.1.1 Binary Equivalent Weight

As an example of what is meant by binary equivalent weight, consider the truth table for the 3-bit
binary signal shown in Figure 12.1. Suppose that we want to change the eight possible digital

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