ADV7128

REV. 0

–6–

Reference Input

An external 1.23 V voltage reference is required to drive the

ADV7128. The AD589 from Analog Devices is an ideal choice

of reference. It is a two-terminal, low cost, temperature com-

pensated bandgap voltage reference which provides a fixed

1.23 V output voltage for input currents between 50

µA and

5 mA. Figure 4 shows a typical reference circuit connection dia-

gram. The voltage reference gets its current drive from the

Ω resistor to the V

REF

pin. A 0.1

µF ceramic capacitor is required between the COMP

the internal reference amplifier.

mines the amplitude of the output video level according to the

following equation:

(1)

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

fixed 560

Ω R

quoted in the specification page. These values typically corre-

spond to the RS-343A video waveform values as shown in

Figure 3.

TO DAC

GND

1k

Ω

RSET

SET

R

560

Ω

500

Ω

100

Ω

*ADDITIONAL CIRCUITRY, INCLUDING DECOUPLING COMPONENTS,

EXCLUDED FOR CLARITY

ANALOG POWER PLANE

COMP

0.01

µF

5V

+

5mA

I

~

AD589

(1.235V

VOLTAGE

REFERENCE)

ADV7128

Figure 4. Reference Circuit

D/A Converter

The ADV7128 contains a 10-bit D/A converter. The DAC is

designed using an advanced, high speed, segmented architec-

ture. 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. The use of identical

current sources in a monolithic design guarantees monotonicity

and low glitch. The on-board operational amplifier stabilizes the

full-scale output current against temperature and power supply

variations.

Analog Output

The analog output of the ADV7128 is a high impedance current

source. The current output 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 the output con-

nected into a doubly terminated 75

Ω load. This arrangement

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

Ω

monitor.

DAC

I

OUT

Z

O

= 75

Ω

(CABLE)

Z

S

= 75

Ω

(SOURCE

TERMINATION)

Z

L

= 75

Ω

(MONITOR)

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

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

Ω

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

DAC remains unchanged, but the source termination resistance,

Ω to 150 Ω.

DAC

I

OUT

Z

O

= 75

Ω

(CABLE)

Z

S

= 150

Ω

(SOURCE

TERMINATION)

Z

L

= 75

Ω

(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 no. E1228-15-1/89.

Figure 3 shows the video waveforms associated with the current

output driving the doubly terminated 75

Ω load of Figure 5a.

Gray Scale Operation

The ADV7128 can be used for stand-alone, gray scale (mono-

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

used for video information).

Video Output Buffer

The ADV7128 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 current 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 recommended. More information on line driver buff-

ering 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.