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AD725 Datasheet(PDF) 12 Page - Analog Devices

Part No. AD725
Description  Low Cost RGB to NTSC/PAL Encoder with Luma Trap Port
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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 12 page
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REV. 0
–12–
AD725
system, the internal 4FSC (14.318 180 MHz) clock that drives
the VGA controller can be used for 4FSC on the AD725. This
signal is not directly accessible from outside the computer, but it
does appear on the VGA card. (A 1FSC-input encoder, the
AD724, is also available.)
If a separate RGB monitor is also to be used, it is not possible to
simply connect it to the R, G and B signals. The monitor pro-
vides a termination that would double terminate these signals.
The R, G, and B signals should be buffered by three amplifiers
with high input impedances. These should be configured for a
gain of two, which is normalized by the divide by two termina-
tion scheme used for the RGB monitor.
The AD8073 is a low cost triple video amplifier that can pro-
vide the buffering required in this application. However, since
the R, G and B signals go all the way to ground during horizon-
tal sync, the AD8073 will require a –5 V supply to handle these
signals. To be able to buffer the R, G and B signals using a
single supply, a rail-to-rail amplifier is required. In this applica-
tion, the AD8051 (single) and AD8052 (dual) can be used to
provide the three required channels. These can be operated on a
single supply of 3 V to 5 V.
Displaying VGA Output on a TV
The AD725 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 AD725 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).
Figure 19 shows a circuit for connection to the VGA port of a
PC. The RGB outputs are ac coupled to the respective inputs of
the AD725. These signals should each be terminated to ground
with 75
Ω.
The standard 15-pin VGA connector has HSYNC on Pin 13
and VSYNC on Pin 14. These signals also connect directly to
the same name signals on the AD725. For a synchronous NTSC
75
75
–5V
4FSC
+5V
0.1 F
+5V (VAA)
AGND
DGND
AD725
CE
RIN
GIN
BIN
HSYNC
VSYNC
CRMA
LUMA
CMPS
APOS
DPOS
0.1 F
10 F
0.1 F
10 F
75
75
75
220 F
COMPOSITE
VIDEO
75
220 F
75
220 F
Y
C
S-VIDEO
(Y/C VIDEO)
75
75
75
B
G
R
RGB MONITOR
VSYNC
HSYNC
FROM VGA PORT
+5V
POWER DOWN
75
+5V
VGA OUTPUT
CONNECTOR
14
15
16
6
7
8
9
10
11
4
5
1k
1k
13
2
0.1 F
0.1 F
0.1 F
OSC
4FSC CLOCK
14.318180MHz (NTSC)
OR
17.734475MHz (PAL)
1/3
AD8073
75
1k
1k
1/3
AD8073
1/3
AD8073
1k
1k
68 H
STND
YTRAP
18pF
9pF
0.1 F
47k
1N4148
NTSC/
PAL
0.1 F
Figure 19. Interfacing the AD725 to the (Interlaced) VGA Port of a PC




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