REV. B

–10–

AD724

220 F

75

75

75

–5V

FIN

OSC

0.1 F

+5V

**

CRYSTAL

0.1 F

*

AGND

DGND

AD724

ENCD

RIN

GIN

BIN

HSYNC

VSYNC

SELECT

STND

CRMA

LUMA

CMPS

APOS

DPOS

10–30pF

0.1 F

10 F

0.1 F

10 F

75

75

JMP

CRYSTAL; 3.579545MHz (NTSC)

OR 4.433620MHz (PAL)

CAPACITOR VALUE DEPENDS ON

CRYSTAL CHOSEN

75

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

+5V

10k

+5V

10k

SELECT

75

JMP

+5V

VGA OUTPUT

CONNECTOR

***

649

649

649

649

649

649

14.31818MHz (NTSC) OR 4.433620MHz,

17.734480MHz(PAL)

1/3

AD8013

1/3

AD8013

1/3

AD8013

0.1 F

0.1 F

0.1 F

Figure 15. Interfacing the AD724 to the (Interlaced) VGA Port of a PC

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.

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).

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-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

Ω series

resistor is required close to each AD724 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 for NTSC (25 MHz for PAL) that must be