0195136047.pdf

308 DIGITAL BUILDING BLOCKS AND COMPUTER SYSTEMS

Vo=−Vref(b 0 × 2 −^1 +b 1 × 2 −^2 +···+bn− 1 × 2 −n) (6.2.3)

Because only two resistor values (Rand 2R) are used, theR–2Rladder converter networks are

relatively simple to manufacture, fast, practical, and reliable. The commercially available AD558,

which is an 8-bitR-2RD/A converter, is an example.

2 n-R D/A CONVERTER

Ann-bit 2n–RD/A converter needs 2nresistors of equal valueRand (2n+^1 −2) analog switches. A

3-bit 2n−RD/A converter is shown in Figure 6.2.11, which includes the eight resistors connected

in series to form a voltage divider providing eight analog voltage levels, as well as 14 analog

switches controlled by the digital input code such that each code creates a single path from the

voltage divider to the converter output. A unit-gain amplifier is connected to the output in order

to prevent loading of the voltage divider. 2n–RD/A converters are economically manufactured as

LSI packages in spite of the large number of components needed.

Analog-to-Digital (A/D) Converters

An A/D converter converts analog input signals into digital output data in many areas such as

process control, aircraft control, and telemetry. Being the interface between analog systems and

digital systems, it plays a key role in many industrial, commercial, and military systems. Several

types of A/D converters exist:counter-controlled, successive-approximation,anddual-ramp

(dual-slope) converters.One should understand its basic operation because analog comparators

form the basis of A/D converters. Figure 6.2.12 shows the block diagram of an analog comparator.

The commercially available LM311 is an example that is widely used by designers.

COUNTER-CONTROLLEDA/D CONVERTER

Figure 6.2.13 shows the block diagram of a counter-controlled A/D converter. Resetting the binary

counter to zero produces D/A output voltageV 2 =0 and initiates the analog-to-digital conversion.

When the analog inputV 1 is larger than the DAC (D/A converter) output voltage, the comparator

output will be high, thereby enabling the AND gate and incrementing the counter.V 2 is increased

101

MSB LSB

0 1 0 1 0

I Vo

in

Vref

b 0 b 1 b 2 b 3

2 R

X RRR

2 R 2 R 2 R 2 R

R

+

−

Figure 6.2.104-bitR–2Rladder D/A converter.