AD8009
75
I
OUT
R
75
RED
75
COAX
PRIMARY MONITOR
ADV7160/
ADV7162
I
OUT
G
75
GREEN
75
I
OUT
B
75
BLUE
75
5V
+
0.1 F
3
7
10 F
75
COAX
RED
75
ADDITIONAL MONITOR
AD8009
2
301
301
6
4
75
0.1 F
–5V
+
10 F
3
AD8009
2
301
301
3
6
75
GREEN
75
AD8009
2
6
75
BLUE
75
301
301
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.
–12–
REV. F
AD [ ANALOG DEVICES ]
