REV. 0

–6–

AD720/AD721

The filtered output is summed with the luminance signal to cre-

ate a composite video signal. The separate luminance, chromi-

nance, and composite video signals are amplified by gain-of-two

amplifiers for driving 75

Ω reverse-terminated lines. The sepa-

rate luminance and chrominance outputs together are known as

“S-Video.”

The digital section of the AD720/AD721 is clocked by the

4FSC input. It measures the width of pulses in the composite

sync input to separate vertical, horizontal, and serration pulses

and to insert the subcarrier burst only after a valid horizontal

sync pulse.

Asserting the ENCD pin to a logical low routes the AD721’s

RGB inputs through three gain-of-two bypass buffers for driving

75

Ω reverse-terminated lines, bypassing the encoder section of

the AD721. The triple bypass amplifier is utilized to overcome

the loading effects of a “TV-out” connection on the RGB moni-

tor output. When a video encoder is connected to outputs of a

current-out video RAMDAC or VGA controller, the R, G, and

B signals to the monitor are loaded-down. This requires the use

of a gain block to properly drive the monitor.

0.1

µF

4

3

2

1

28

27

26

18

17

16

15

14

13

12

11

10

9

8

6

7

5

19

25

24

23

22

21

20

AGND

VNEG

SYNC

DPOS

4FSC

DPOS

ASNC

DGND

ENCD

RDIN

AGND

GRIN

AGND

BLIN

STND

AGND

VNEG

DGND

AD720

CHROMINANCE OUTPUT

COMPOSITE OUTPUT

LUMINANCE OUTPUT

75

Ω

75

Ω

75

Ω

RGB TO NTSC/PAL

ENCODER

0.1

µF

0.1

µF

0.1

µF

0.1

µF

0.1

µF

–5V FROM ANALOG SUPPLY

COMPOSITE SYNC INPUT

CMOS LOGIC LEVEL

NEGATIVE SYNC TIPS

+5V FROM

DIGITAL SUPPLY

+5V FROM

DIGITAL SUPPLY

4 X SUBCARRIER INPUT

CMOS LOGIC LEVELS

NTCS = 14.318 180MHz

PAL = 17.734 480MHz

ENCODE INPUT

ENCODE = CMOS HIGH

POWER DOWN = CMOS LOW

VIDEO STANDARD

SELECTION INPUT

NTSC = CMOS HIGH

PAL = CMOS LOW

+5V FROM

ANALOG SUPPLY

+5V FROM

ANALOG SUPPLY

–5V FROM

ANALOG SUPPLY

75

Ω

75

Ω

75

Ω

VIDEO

RAM-DAC

ADV47X

ADV71XX

IOR

IOG

IOB

75

Ω

75

Ω

75

Ω

Figure 9. AD720 Application

+5V FROM

ANALOG SUPPLY

0.1

µF

0.1

µF

–5V FROM

ANALOG SUPPLY

BLUE OUTPUT

GREEN OUTPUT

RED OUTPUT

75

Ω

75

Ω

75

Ω

4

3

2

1

28

27

26

18

17

16

15

14

13

12

11

10

9

8

6

7

5

19

25

24

23

22

21

20

AGND

VNEG

SYNC

DPOS

4FSC

DPOS

ASNC

DGND

ENCD

RDIN

AGND

GRIN

AGND

BLIN

STND

AGND

VNEG

DGND

AD721

CHROMINANCE OUTPUT

COMPOSITE OUTPUT

LUMINANCE OUTPUT

75

Ω

75

Ω

75

Ω

RGB TO NTSC/PAL

ENCODER

0.1

µF

0.1

µF

0.1

µF

0.1

µF

–5V FROM ANALOG SUPPLY

COMPOSITE SYNC INPUT

CMOS LOGIC LEVEL

NEGATIVE SYNC TIPS

+5V FROM

DIGITAL SUPPLY

+5V FROM

DIGITAL SUPPLY

4 X SUBCARRIER INPUT

CMOS LOGIC LEVELS

NTCS = 14.318 180MHz

PAL = 17.734 480MHz

ENCODE INPUT

ENCODE = CMOS HIGH

BYPASS = CMOS LOW

VIDEO STANDARD

SELECTION INPUT

NTSC = CMOS HIGH

PAL = CMOS LOW

+5V FROM

ANALOG SUPPLY

75

Ω

75

Ω

75

Ω

VIDEO

RAM-DAC

ADV47X

ADV71XX

IOR

IOG

IOB

75

Ω

75

Ω

75

Ω

Figure 10. AD721 Application