AD722 View Datasheet(PDF) - Analog Devices
Part Name
Description
Manufacturer
AD722 Datasheet PDF : 12 Pages
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AD722
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
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).
6
1
11
7
2
12
8
3
13
9
4
14
10
5
15
SELECT
VGA OUTPUT
CONNECTOR
75Ω
+5V
0.1µF
+
10µF 0.1µF
75Ω 75Ω
+5V
10kΩ
JMP
OSC * *
*
CRYSTAL
10–30pF
+5V
10kΩ
JMP
+5V (VAA)
AD813
0.1µF
5 11
75Ω
6 1/3 7
1
649Ω 649Ω
12
14
75Ω
13 1/3
2
649Ω 649Ω
10
8
75Ω
9 1/3 4
3
649Ω 649Ω –5V
+5V
+5V
4
APOS
5 ENCD
14
DPOS
6 RIN
CMPS 10
7 GIN AD722
8 BIN
16 HSYNC
LUMA 11
15 VSYNC
CRMA 9
12 SELECT
+
0.1µF 10µF
+ 75Ω
220µF
+ 75Ω
220µF
+ 75Ω
220µF
COMPOSITE
VIDEO
Y
C
1 STND
S-VIDEO
(Y/C VIDEO)
3 FIN
AGND DGND
2
13
* PARALLEL–RESONANT
CRYSTAL; 3.579545MHz (NTSC)
OR 4.433620MHz (PAL)
CAPACITOR VALUE DEPENDS ON
CRYSTAL CHOSEN
VSYNC
HSYNC
**FSC OR 4FSC CLOCK; 3.579545MHz,
14.31818MHz (NTSC) OR 4.433620MHZ,
17.734480MHz(PAL)
FROM VGA PORT
B
75Ω
G
75Ω
R
75Ω
RGB MONITOR
REV. 0
Figure 15. Interfacing the AD722 to the (Interlaced) VGA Port of a PC
–9–
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