AD722

REV. 0

–9–

resistor is required close to each AD722 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 that must be 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 frequency and can use a smaller ca-

pacitor if desired, but 220

µF can be used to minimize the num-

ber of different components used in the design.

Displaying VGA Output on a TV

The AD722 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 AD722 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 sig-

nal 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).

Basically these two standards use most of the features of the

standard that their names imply, but use the subcarrier that is

equal to or approximately equal to the frequency of the other

standard. Because of the automatic programming of the filters in

the chrominance path and other timing considerations, it is not

possible to support these standards.

Layout Considerations

The AD722 is an all CMOS mixed signal part. It has separate

pins for the analog and digital +5 V and ground power supplies.

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-thru

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 AD722 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 AD722 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

Figure 15. Interfacing the AD722 to the (Interlaced) VGA Port of a PC

0.1µF

75

Ω

649

Ω

649

Ω

75

Ω

649

Ω

649

Ω

75

Ω

649

Ω

649

Ω

AD813

5

6

7

11

12

13

14

9

10

8

1/3

1/3

–5V

1

2

3

4

13

2

FIN

OSC

0.1µF

+5V

**

*

CRYSTAL

AGND

DGND

14

15

7

9

10

11

1

4

5

AD722

ENCD

RIN

GIN

BIN

HSYNC

VSYNC

SELECT

STND

CRMA

LUMA

CMPS

APOS

DPOS

+

10–30pF

0.1µF

10µF

0.1µF

10µF

75

Ω

75

Ω

JMP

CRYSTAL; 3.579545MHz (NTSC)

OR 4.433620MHz (PAL)

CAPACITOR VALUE DEPENDS ON

CRYSTAL CHOSEN

14.31818MHz (NTSC) OR 4.433620MHZ,

17.734480MHz(PAL)

+

75

Ω

220µF

COMPOSITE

VIDEO

+

75

Ω

220µF

+

75

Ω

220µF

Y

C

S-VIDEO

(Y/C VIDEO)

75

Ω

75

Ω

75

Ω

B

G

R

RGB MONITOR

VSYNC

HSYNC

FROM VGA PORT

+5V

+5V

10k

Ω

+5V

10k

Ω

SELECT

75

Ω

1/3

JMP

12

3

16

8

6

6

7

8

9

10

11

12

13

14

15

+

+5V

1

2

3

4

5

VGA OUTPUT

CONNECTOR