AD724
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.
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 fre-
quency and can use a smaller capacitor if desired, but 220 µF
can be used to minimize the number of different components
used in the design.
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-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.
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.
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 Ω series
resistor is required close to each AD724 output, while 75 Ω to
ground should terminate the far end of each line.
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).
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
+5V
+5V
SELECT
0.1F
10F
10F
0.1F
DPOS
APOS
ENCD
***
RIN
75⍀
0.1F
0.1F
COMPOSITE
VIDEO
CMPS
GIN
220F
AD724
BIN
0.1F
HSYNC
VSYNC
75⍀
75⍀
Y
LUMA
CRMA
+5V
75⍀ 75⍀
75⍀
220F
220F
10k⍀
JMP
+5V
10k⍀
C
SELECT
STND
JMP
+5V
**
OSC
S-VIDEO
(Y/C VIDEO)
FIN
AGND
DGND
0.1F
+5V (V
)
CRYSTAL
10–30pF
AA
*
0.1F
*
PARALLEL–RESONANT
CRYSTAL; 3.579545MHz (NTSC)
OR 4.433620MHz (PAL)
CAPACITOR VALUE DEPENDS ON
CRYSTAL CHOSEN
VGA OUTPUT
CONNECTOR
75⍀
1/3
AD8013
**FSC OR 4FSC CLOCK; 3.579545MHz,
14.31818MHz (NTSC) OR 4.433620MHz,
17.734480MHz(PAL)
649⍀
649⍀
*** 0.1F CAPACITORS RECOMMENDED
VSYNC
75⍀
1/3
AD8013
FROM VGA PORT
HSYNC
B
75⍀
649⍀
649⍀
G
75⍀
75⍀
1/3
AD8013
R
75⍀
–5V
649⍀
649⍀
RGB MONITOR
Figure 15. Interfacing the AD724 to the (Interlaced) VGA Port of a PC
–10–
REV. B
ADI [ ADI ]
