AD8009

–12–

REV. F

10 F

+

0.1 F

AD8009

75

301

5V

301

2

7

3

6

+

10 F

0.1 F

4

–5V

AD8009

75

301

301

2

3

6

AD8009

75

301

301

2

3

6

75

COAX

PRIMARY MONITOR

ADDITIONAL MONITOR

75

COAX

75

75

75

75

75

75

75

75

75

RED

GREEN

BLUE

RED

GREEN

BLUE

ADV7160/

ADV7162

Figure 4. Driving an Additional High Resolution Monitor Using Three AD8009s

RGB Monitor Driver

High resolution computer monitors require very high full power

bandwidth signals to maximize their display resolution. The

RGB signals that drive these monitors are generally provided by

a current-out RAMDAC that can directly drive a 75

Ω doubly

terminated line.

There are times when the same output wants to be delivered to

additional monitors. The termination provided internally by

each monitor prohibits the ability to simply connect a second

monitor in parallel with the first. Additional buffering must be

provided.

Figure 4 shows a connection diagram for two high resolution

monitors being driven by an ADV7160 or ADV7162, a 220 MHz

(Megapixel per second) triple RAMDAC. This pixel rate

requires a driver whose full power bandwidth is at least half the

pixel rate or 110 MHz. This is to provide good resolution for a

worst-case signal that swings between zero scale and full scale

on adjacent pixels.

The primary monitor is connected in the conventional fashion

with a 75

Ω termination to ground at each end of the 75 Ω

cable. Sometimes this configuration is called “doubly termi-

nated” and is used when the driver is a high output impedance

current source.

For the additional monitor, each of the RGB signals close to the

RAMDAC output is applied to a high input impedance, noninvert-

ing input of an AD8009 that is configured for a gain of +2. The

outputs each drive a series 75

Ω resistor, cable, and termination

resistor in the monitor that divides the output signal by two, thus

providing an overall unity gain. This scheme is referred to as

“back termination” and is used when the driver is a low output

impedance voltage source. Back termination requires that the

voltage of the signal be double the value that the monitor sees.

Double termination requires that the output current be double the

value that flows in the monitor termination.