ADV7127

–13–

REV. 0

D/A Converter

The ADV7127 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 ADV7127 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 22 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.

(CABLE)

(SOURCE

TERMINATION)

(MONITOR)

DAC

Figure 22. Analog Output Termination for RS-343A

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

Ω

monitor is shown in Figure 23. The output current level of the

DAC remains unchanged, but the source termination resistance,

Ω to 150 Ω.

(CABLE)

(SOURCE

TERMINATION)

(MONITOR)

DAC

Figure 23. 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 &

Required Load Terminations” available from Analog Devices,

publication no. E1228-15-1/89.

Figure 21 shows the video waveforms associated with the current

output driving the doubly terminated 75

Ω load of Figure 22.

Gray Scale Operation

The ADV7127 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 ADV7127 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 23. 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 two

and four 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

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.

AD848

0.1 F

(CABLE)

(SOURCE

TERMINATION)

(MONITOR)

DAC

75

0.1 F

GAIN (G) = 1 +

Figure 24. AD848 As an Output Buffer

PC Board Layout Considerations

The ADV7127 is optimally designed for lowest noise perfor-

mance, both radiated and conducted noise. To complement the

excellent noise performance of the ADV7127 it is imperative

that great care be given to the PC board layout. Figure 25 shows

a recommended connection diagram for the ADV7127.

The layout should be optimized for lowest noise on the ADV7127

power and ground lines. This can be achieved by shielding the

digital inputs and providing good decoupling. The lead length

inductive ringing.

Ground Planes

The ADV7127 and associated analog circuitry, should have a

separate ground plane referred to as the analog ground plane.

This ground plane should connect to the regular PCB ground

plane at a single point through a ferrite bead, as illustrated in

Figure 25. This bead should be located as close as possible

(within 3 inches) to the ADV7127.

The analog ground plane should encompass all ADV7127

ground pins, voltage reference circuitry, power supply bypass

circuitry, the analog output traces and any output amplifiers.

The regular PCB ground plane area should encompass all the

digital signal traces, excluding the ground pins, leading up to

the ADV7127.