AD8044
+5V
1000
GRAPHICS
IC
V
= +5V
S
R
< 30% OVERSHOOT
⍀
= 10
⍀
= 0
S
R
75⍀
S
R
G
75⍀
100
B
75⍀
75⍀
75⍀
R
R
F
G
R
S
V
RGB
MONITOR #1
IN
+3V OR +5V
75⍀
V
OUT
C
100mV STEP
L
AD8044
10
1
10F
0.1F
75⍀
2
3
4
5
6
A
– V/V
CL
V+
A
B
C
Figure 35. Capacitive Load Drive vs. Closed-Loop Gain
1k⍀
APPLICATIONS
RGB Buffer
The AD8044 can provide buffering of RGB signals that include
ground while operating from a single +3 V or +5 V supply.
1k⍀
75⍀
75⍀
AD8044
75⍀
When driving two monitors from the same RGB video source it
is necessary to provide an additional driver for one of the moni-
tors to prevent the double termination situation that the second
monitor presents. This has usually required a dual supply op
amp because the level of the input signal from the video driver
goes all the way to ground during horizontal blanking. In single
supply systems it can be a major inconvenience and expense to
add an additional negative supply.
1k⍀
1k⍀
75⍀
AD8044
V–
75⍀
RGB
MONITOR #2
1k⍀
A single AD8044 can provide the necessary drive capability and
yet does not require a negative supply in this application. Fig-
ure 36 is a schematic that uses three amplifiers out of a single
AD8044 to provide buffering for a second monitor.
1k⍀
Figure 36. Single Supply RGB Video Driver
The source of the RGB signals is shown to be from a set of three
current output DACs that are within a single supply graphics
IC. This is typically the situation in most PCs and workstations
which might use either a stand-alone triple DAC or DACs that
are integrated into a larger graphics chip.
Figure 37 is an oscilloscope photo of the circuit in Figure 36
operating from a +3 V supply and driven by the Blue signal of a
color bar pattern. Note that the input and output are at ground
during the horizontal blanking interval. The RGB signals are
specified to output a maximum of 700 mV peak. The output of
the AD8044 is 1.4 V with the termination resistors providing a
divide-by-two.
During horizontal blanking, the current output from the DACs
is turned off and the RGB outputs are pulled to ground by the
termination resistors. If voltage sources were used for the RGB
signals, then the termination resistors near the graphics IC
would be in series and the rest of the circuit would remain the
same. This is because a voltage source is an ac short circuit, so a
series resistor is required to make the drive end of the line see
75 Ω to ac ground. On the other hand, a current source has a
very high output impedance, so a shunt resistor is required to
make the drive end of the line see 75 Ω to ground. In either
case, the monitor terminates its end of the line with 75 Ω.
500mV
5s
100
90
V
IN
GND
GND
V
OUT
10
The circuit of Figure 36 shows minimum signal degradation
when using a single supply for the AD8044. The circuit per-
forms equally well on either a +3 V or +5 V supply.
0%
500mV
Figure 37. +3 V, RGB Buffer
–12–
REV. A
ADI [ ADI ]
