ADV7120

REV. B

–8–

determines the amplitude of the output video level according to

the following equations:

IOG (mA) = 12,082

× V

Ω)

(1)

IOR, IOB (mA) = 8,628

× V

(2)

If SYNC is not being encoded onto the green channel, then

Equation 1 will be similar to Equation 2.

accurate adjustment of the analog output video levels. Use of a

fixed 560

Ω R

quoted in the specification page. These values also correspond

to the RS-343A video waveform values as shown in Figure 3.

TO DACs

ADV7120*

V

AA

V

REF

GND

1k

Ω

FS ADJUST

R

SET

560

Ω

500

Ω

100

Ω

*ADDITIONAL CIRCUITRY, INCLUDING

DECOUPLING COMPONENTS,

EXCLUDED FOR CLARITY

ANALOG POWER PLANE

COMP

0.1

µF

5V

+

AD589

(1.235V

VOLTAGE

REFERENCE)

I

Figure 4. Reference Circuit

D/A Converters

The ADV7120 contains three matched 8-bit D/A converters.

The DACs are designed using an advanced, high speed, seg-

mented architecture. The bit currents corresponding to each

digital input are routed to either the analog output (bit = “1”)

or GND (bit = “0”) by a sophisticated decoding scheme. As all

this circuitry is on one monolithic device, matching between the

three DACs is optimized. As well as matching, the use of identi-

cal current sources in a monolithic design guarantees monoto-

nicity and low glitch. The onboard operational amplifier

stabilizes the full-scale output current against temperature and

power supply variations.

Analog Outputs

The ADV7120 has three analog outputs, corresponding to the red,

used if it is required to encode video synchronization information

(See “Video Synchronization and Control” section.)

The red, green and blue analog outputs of the ADV7102 are

high impedance current sources. Each one of these three RGB

current outputs is capable of directly driving a 37.5

Ω load, such

as a doubly terminated 75

Ω coaxial cable. Figure 5a shows the

required configuration for each of the three RGB outputs con-

nected into a doubly terminated 75

Ω load. This arrangement

will develop RS-343A video output voltage levels across a 75

Ω

monitor.

TERMINATION REPEATED THREE TIMES

FOR RED, GREEN AND BLUE DACs

DACs

IOR, IOG, IOB

(CABLE)

Z

Z

(SOURCE

TERMINATION)

Z

(MONITOR)

Figure 5a. Analog Output Termination for RS-343A

One suggested method of driving RS-170 video levels into a 75

Ω

monitor is shown in Figure 5b. The output current levels of the

DACs remain unchanged but the source termination resistance,

Ω to 150 Ω.

TERMINATION REPEATED THREE TIMES

FOR RED, GREEN AND BLUE DACs

DACs

IOR, IOG, IOB

(CABLE)

Z

Z

(SOURCE

TERMINATION)

Z

(MONITOR)

Figure 5b. Analog Output Termination for RS-170

More detailed information regarding load terminations for vari-

ous output configurations, including RS-343A and RS-170, is

available in an application note entitled “Video Formats & Re-

quired Load Terminations” available from Analog Devices,

publication number E1228-15-1/89.

Figure 3 shows the video waveforms associated with the three

RGB outputs driving the doubly terminated 75

Ω load of Figure

5a. As well as the gray scale levels, black level to white level, the

diagram also shows the contributions of SYNC and BLANK.

These control inputs add appropriately weighted currents to the

analog outputs, producing the specific output level requirements

for video applications. Table I details how the SYNC and

BLANK

inputs modify the output levels.

Gray Scale Operation

The ADV7120 can be used for stand-alone, gray scale (mono-

chrome) or composite video applications (i.e., only one channel

used for video information). Any one of the three channels, red,

green or blue, can be used to input the digital video data. The

two unused video data channels should be tied to logical zero.