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AD722 Datasheet(PDF) 9 Page - Analog Devices

Part No. AD722
Description  RGB to NTSC/PAL Encoder
Download  12 Pages
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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AD722
REV. 0
–9–
resistor is required close to each AD722 output, while 75
Ω to
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 that must be transmitted. Each output requires a
220
µF series capacitor to work with the 75 Ω resistance to pass
these low frequencies. The CRMA signal has information
mostly up at the chroma frequency and can use a smaller ca-
pacitor if desired, but 220
µF can be used to minimize the num-
ber of different components used in the design.
Displaying VGA Output on a TV
The AD722 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 AD722 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 sig-
nal 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).
Basically these two standards use most of the features of the
standard that their names imply, but use the subcarrier that is
equal to or approximately equal to the frequency of the other
standard. Because of the automatic programming of the filters in
the chrominance path and other timing considerations, it is not
possible to support these standards.
Layout Considerations
The AD722 is an all CMOS mixed signal part. It has separate
pins for the analog and digital +5 V and ground power supplies.
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
µF capacitor and a larger tantalum capacitor of about 10 µF.
The three analog inputs (RIN, GIN, BIN) should be terminated
with 75
Ω to ground close to the respective pins. However, as
these are high impedance inputs, they can be in a loop-thru
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 AD722 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 AD722 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
Ω series
Figure 15. Interfacing the AD722 to the (Interlaced) VGA Port of a PC
+5V (VAA)
0.1µF
75
649
649
75
649
649
75
649
649
AD813
5
6
7
11
12
13
14
9
10
8
1/3
1/3
–5V
1
2
3
4
13
2
FIN
OSC
0.1µF
+5V
**
*
CRYSTAL
AGND
DGND
14
15
7
9
10
11
1
4
5
AD722
ENCD
RIN
GIN
BIN
HSYNC
VSYNC
SELECT
STND
CRMA
LUMA
CMPS
APOS
DPOS
+
10–30pF
0.1µF
10µF
0.1µF
10µF
75
75
JMP
* PARALLEL–RESONANT
CRYSTAL; 3.579545MHz (NTSC)
OR 4.433620MHz (PAL)
CAPACITOR VALUE DEPENDS ON
CRYSTAL CHOSEN
**FSC OR 4FSC CLOCK; 3.579545MHz,
14.31818MHz (NTSC) OR 4.433620MHZ,
17.734480MHz(PAL)
+
75
220µF
COMPOSITE
VIDEO
+
75
220µF
+
75
220µF
Y
C
S-VIDEO
(Y/C VIDEO)
75
75
75
B
G
R
RGB MONITOR
VSYNC
HSYNC
FROM VGA PORT
+5V
+5V
10k
+5V
10k
SELECT
75
1/3
JMP
12
3
16
8
6
6
7
8
9
10
11
12
13
14
15
+
+5V
1
2
3
4
5
VGA OUTPUT
CONNECTOR




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