AD724JR View Datasheet(PDF) - Analog Devices

Part Name

Description

Manufacturer

AD724JR

RGB to NTSC/PAL Encoder

Analog Devices 

AD724

Both the analog and digital ground pins should be tied to the

ground plane by a short, low inductance path. Each power

transmitted. Each output requires a 220 µF series capacitor to

work with the 75 Ω resistance to pass these low frequencies. The

supply pin should be bypassed to ground by a low inductance

CRMA signal has information mostly up at the chroma fre-

0.1 µF capacitor and a larger tantalum capacitor of about 10 µF. quency and can use a smaller capacitor if desired, but 220 µF

The three analog inputs (RIN, GIN, BIN) should be terminated

with 75 Ω to ground close to the respective pins. However, as

can be used to minimize the number of different components

used in the design.

these are high impedance inputs, they can be in a loop-through Displaying VGA Output on a TV

configuration. This technique is used to drive two or more

The AD724 can be used to convert the analog RGB output from a

devices with high frequency signals that are separated by some

personal computer’s VGA card to the NTSC or PAL television

distance. A connection is made to the AD724 with no local

standards. To accomplish this it is important to understand that

termination, and the signals are run to another distant device

the AD724 requires interlaced RGB video and clock rates that

where the termination for these signals is provided.

are consistent with those required by the television standards. In

The output amplitudes of the AD724 are double that required

by the devices that it drives. This compensates for the halving of

most computers the default output is a noninterlaced RGB

signal at a frame rate higher than used by either NTSC or PAL.

the signal levels by the required terminations. A 75 Ω series

resistor is required close to each AD724 output, while 75 Ω to

Most VGA controllers support a wide variety of output modes

that are controlled by altering the contents of internal registers.

ground should terminate the far end of each line.

It is best to consult with the VGA controller manufacturer to

The outputs have a dc bias and must be ac coupled for proper

operation. The COMP and LUMA outputs have information

determine the exact configuration required to provide an inter-

laced output at 60 Hz (50 Hz for PAL).

down to 30 Hz for NTSC (25 MHz for PAL) that must be

SELECT

+5V

+5V

10␮F 0.1␮F

0.1␮F 10␮F

APOS DPOS

+5V

0.1␮F

75⍀ 75⍀

75⍀

+5V

JMP 10k⍀

OSC * *

* CRYSTAL

10–30pF

***

0.1␮F

0.1␮F

0.1␮F

+5V

JMP 10k⍀

ENCD

RIN

GIN

CMPS

BIN AD724

HSYNC

VSYNC

LUMA

SELECT

CRMA

+5V (VAA)

0.1␮F

STND

FIN AGND DGND

75⍀

220␮F

75⍀

220␮F

75⍀

220␮F

COMPOSITE

VIDEO

Y

C

S-VIDEO

(Y/C VIDEO)

VGA OUTPUT

CONNECTOR

1/3

AD8013

649⍀

649⍀

75⍀

* 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)

*** 0.1␮F CAPACITORS RECOMMENDED

1/3

75⍀

AD8013

VSYNC

FROM VGA PORT

HSYNC

649⍀

649⍀

B

75⍀

G

75⍀

1/3

75⍀

AD8013

R

75⍀

649⍀

–5V

649⍀

RGB MONITOR

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

–10–

REV. B

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