AD724 Analog Devices, AD724 Datasheet - Page 10

• Manufacturer Part Number: AD724
• Description: RGB to NTSC/PAL Encoder
• Manufacturer: Analog Devices

Both the analog and digital ground pins should be tied to the
ground plane by a short, low inductance path. Each power
supply pin should be bypassed to ground by a low inductance
0.1 F capacitor and a larger tantalum capacitor of about 10 F.
The three analog inputs (RIN, GIN, BIN) should be terminated
with 75
these are high impedance inputs, they can be in a loop-through
configuration. This technique is used to drive two or more
devices with high frequency signals that are separated by some
distance. A connection is made to the AD724 with no local
termination, and the signals are run to another distant device
where the termination for these signals is provided.
The output amplitudes of the AD724 are double that required
by the devices that it drives. This compensates for the halving of
the signal levels by the required terminations. A 75
resistor is required close to each AD724 output, while 75
ground should terminate the far end of each line.
The outputs have a dc bias and must be ac coupled for proper
operation. The COMP and LUMA outputs have information
down to 30 Hz for NTSC (25 MHz for PAL) that must be
AD724
VGA OUTPUT
CONNECTOR
to ground close to the respective pins. However, as
SELECT
Figure 15. Interfacing the AD724 to the (Interlaced) VGA Port of a PC
0.1 F
+5V
OSC
75
**
*
75
CRYSTAL
series
75
JMP
649
649
649
to
+5V
10–30pF
10k
–10–
10 F
649
649
649
transmitted. Each output requires a 220 F series capacitor to
work with the 75
CRMA signal has information mostly up at the chroma fre-
quency and can use a smaller capacitor if desired, but 220 F
can be used to minimize the number of different components
used in the design.
Displaying VGA Output on a TV
The AD724 can be used to convert the analog RGB output from a
personal computer’s VGA card to the NTSC or PAL television
standards. To accomplish this it is important to understand that
the AD724 requires interlaced RGB video and clock rates that
are consistent with those required by the television standards. In
most computers the default output is a noninterlaced RGB
signal at a frame rate higher than used by either NTSC or PAL.
Most VGA controllers support a wide variety of output modes
that are controlled by altering the contents of internal registers.
It is best to consult with the VGA controller manufacturer to
determine the exact configuration required to provide an inter-
laced output at 60 Hz (50 Hz for PAL).
AD8013
AD8013
AD8013
+5V (V
1/3
JMP
1/3
1/3
0.1 F
0.1 F
0.1 F
0.1 F
+5V
AA
***
–5V
0.1 F
10k
)
ENCD
RIN
GIN
BIN
HSYNC
VSYNC
SELECT
FIN
STND
75
75
75
APOS
+5V
resistance to pass these low frequencies. The
AGND
AD724
DPOS
+5V
DGND
CMPS
CRMA
LUMA
***
**
*
75
75
75
B
G
CRYSTAL; 3.579545MHz (NTSC)
OR 4.433620MHz (PAL)
CAPACITOR VALUE DEPENDS ON
CRYSTAL CHOSEN
R
14.31818MHz (NTSC) OR 4.433620MHz,
17.734480MHz(PAL)
PARALLEL–RESONANT
FSC OR 4FSC CLOCK; 3.579545MHz,
0.1 F CAPACITORS RECOMMENDED
220 F
220 F
220 F
RGB MONITOR
0.1 F
VSYNC
HSYNC
75
75
75
10 F
FROM VGA PORT
C
COMPOSITE
VIDEO
(Y/C VIDEO)
S-VIDEO
Y
REV. B