ADV7120

REV. B

–9–

The unused analog outputs should be terminated with the same

load as that for the used channel. In other words, if the red

channel is used and IOR is terminated with a doubly terminated

75

Ω load (37.5 Ω), IOB and IOG should be terminated with

37.5

Ω resistors. (See Figure 6.)

GND

ADV7120

R0

R7

G0

G7

B0

B7

VIDEO

INPUT

DOUBLY

TERMINATED

75

Ω LOAD

IOR

IOG

IOB

37.5

Ω

37.5

Ω

Figure 6. Input and Output Connections for Stand-Alone

Gray Scale or Composite Video

Video Output Buffers

The ADV7120 is specified to drive transmission line loads,

which is what most monitors are rated as. The analog output

configurations to drive such loads are described in the Analog

Interface section and illustrated in Figure 5. However, in some

applications it may be required to drive long “transmission line”

cable lengths. Cable lengths greater than 10 meters can attenu-

ate and distort high frequency analog output pulses. The inclu-

sion of output buffers will compensate for some cable distortion.

Buffers with large full power bandwidths and gains between 2

and 4 will be required.

These buffers will also need to be able to supply sufficient cur-

rent over the complete output voltage swing. Analog Devices

produces a range of suitable op amps for such applications.

These include the AD84X series of monolithic op amps. In very

high frequency applications (80 MHz), the AD9617 is recom-

mended. More information on line driver buffering circuits is

given in the relevant op amp data sheets.

Use of buffer amplifiers also allows implementation of other

video standards besides RS-343A and RS-170. Altering the gain

components of the buffer circuit will result in any desired

video level.

7

6

4

3

2

AD848

DACs

IOR, IOG, IOB

(CABLE)

Z

Z

(SOURCE

TERMINATION)

Z

(MONITOR)

0.1

µF

75

Ω

+V

S

Z

1

Z

2

–V

S

0.1

µF

GAIN (G) = 1 +

Z

1

Z

2

Figure 7. AD848 As an Output Buffer