AD725 datasheet(12/20 Pages) AD

12 / 20 page

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

–5V

4FSC

+5V

0.1 F

+5V (VAA)

AGND

DGND

AD725

RIN

GIN

BIN

HSYNC

VSYNC

CRMA

LUMA

CMPS

APOS

DPOS

0.1 F

10 F

0.1 F

10 F

220 F

COMPOSITE

VIDEO

220 F

S-VIDEO

(Y/C VIDEO)

RGB MONITOR

VSYNC

HSYNC

FROM VGA PORT

+5V

POWER DOWN

+5V

VGA OUTPUT

CONNECTOR

0.1 F

OSC

4FSC CLOCK

14.318180MHz (NTSC)

17.734475MHz (PAL)

1/3

AD8073

1/3

AD8073

1/3

AD8073

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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