AD723 datasheet(15/20 Pages) AD

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REV. 0

AD723

–15–

SVGA MONITOR

TELEVISION

I/O

CONTROLLER

HSYNC

VSYNC

RED

GREEN

BLUE

GRAPHICS

SUBSYSTEM

VCC

GPO

GPI

GPO

TERM

TVDET

STND

COMPOSITE

VIDEO

S-VIDEO

(Y/C VIDEO)

AD723

YSET

CSET

CVSET

OPTIONAL

0.1 F

VCC

0.1 F

375

300

375

300

375

300

0.1 F

10 F

DDCSCL 5V

DDCSDL 5V

VCC

ESD

PROTECTION

PI FILTER

Figure 8. PC Interface (Using Switch Input Termination)

TO PIN 17

OF AD723

U1A

U1B

200

60pF

47pF

~15pF

(OPT)

TO OTHER

DEVICE CLOCKS

HC04

Figure 9. Low Cost Crystal Oscillator

The crystal should be a parallel resonant type at the appropriate

frequency (NTSC/PAL, 4FSC). The series combination of C1 and

C2 should be approximately equal to the crystal manufacturer’s

specification for the parallel capacitance required for the crystal

to operate at its specified frequency. C1 will usually want to be a

somewhat smaller value because of the input parasitic capacitance

of the inverter. If it is desired to tune the frequency to greater

accuracy, C1 can be made still smaller and a parallel adjust-

able capacitor can be used to adjust the frequency to the

desired accuracy.

Resistor R2 serves to provide the additional phase shift required

by the circuit to sustain oscillation. It can be sized by R2 =

1/(2

× π × f × C2). Other functions of R2 are to provide a low-

pass filter that suppresses oscillations at harmonics of the

fundamental of the crystal and to isolate the output of the inverter

from the resonant load that the crystal network presents.

The basic oscillator described above is buffered by U1B to drive

the AD723 4FSC pin and other devices in the system. For a

system that requires both an NTSC and PAL oscillator, the

circuit can be duplicated by using a different pair of inverters

from the same package.

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Numerous distortions are apparent in the presentation of com-

posite signals on TV monitors. These effects will vary in degree,

depending on the circuitry used by the monitor to process the

signal and on the nature of the image being displayed. It is

generally not possible to produce pictures on a composite moni-

tor that are as high quality as those produced by standard quality

RGB, VGA monitors.

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