AD723 Analog Devices, AD723 Datasheet - Page 15

• Manufacturer Part Number: AD723
• Description: 2.7 V to 5.5 V RGB-to-NTSC/PAL Encoder with Load Detect and Input Termination Switch
• Manufacturer: Analog Devices

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.
~
(OPT)
15pF
C3
CONTROLLER
SUBSYSTEM
GRAPHICS
I/O
C1
47pF
GPO
GPO
GPI
GPO
GPO
HC04
1M
R1
Y1
RED
GREEN
BLUE
HSYNC
VSYNC
DDCSCL 5V
DDCSDL 5V
U1A
ESD
PROTECTION
OPTIONAL
CE
TERM
TVDET
SA
STND
R2
200
C2
60pF
0.1 F
HC04
V
CC
75
U1B
0.1 F
75
AD723
10 F
0.1 F
0.1 F
0.1 F
0.1 F
TO PIN 17
OF AD723
TO OTHER
DEVICE CLOCKS
+
75
V
CC
CVSET
YSET
CSET
3V
3V
CV
C
Y
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.
Dot Crawl
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.
V
CC
375
300
375
300
375
300
COMPOSITE
VIDEO
PI FILTER
(Y/C VIDEO)
S-VIDEO
TELEVISION
SVGA MONITOR
AD723