0195136047.pdf

6.2 DIGITAL SYSTEM COMPONENTS 315

CATHODE-RAYTUBE(CRT)

While the CRT display is one of the oldest, it still remains one of the most popular display
technologies. Theraster-scanneddisplay devices work similarly to commercial television sets,
whereasgraphicsdisplay devices use different technologies to achieve extremely high resolution.
A CRT video signal has only two levels, with a 0 level causing a dark spot and a 1 level causing
a bright spot. The appropriate combination of 1s and 0s displays data on a CRT screen, with each
character displayed by dot-matrix displays, typically in 5×7or7×9 display fonts.

EXAMPLE 6.2.6

For the 4-bit D/A converter of Figure 6.2.9 withVref=−5 V, determine the range of analog
output voltage and the smallest increment.

Solution

The binary input range is from 0000 to1111 for a4-bit D/A converter. From Equation (6.2.1) it
follows that the range of the analog output voltage is from0Vto

5 ×( 1 + 1 × 2 −^1 + 1 × 2 −^2 + 1 × 2 −^3 )= 5 ×1.875= 9 .375 V

The smallest increment is given by 5× 1 × 2 −^3 = 5 / 8 = 0 .625 V.

EXAMPLE 6.2.7

For the 3-bit 2n–RD/A converter of Figure 6.2.11, calculate the analog output voltage when the
input is (a) 100, and (b) 010.

Solution

a. For the binary input 100, switches controlled byB 0 ,B ̄ 1 , andB ̄ 2 will be closed. A path

is then produced between the outputVoand point 4, where the voltage is equal toVref/2.

The analog output voltage is thereforeVref/2.

b. For the binary input 010, switches controlled byB ̄ 0 ,B 1 , andB ̄ 2 will be closed. A path

exists betweenVoand point 6, where the voltage is equal toVref/4. The analog output

voltage is thusVref/4.

EXAMPLE 6.2.8

The speed of an 8-bit A/D converter is limited by the counter, which has a maximum speed of
4 × 107 counts per second. Estimate the maximum number of A/D conversions per second that
can be achieved.