AD8044
1000
+5V
GRAPHICS
IC
V
S
= +5V
< 30% OVERSHOOT
R
CAPACITIVE LOAD (pF)
R
S
0
=1
R
S
=0
G
75
B
75
75
100
R
G
V
IN
100mV STEP
10
1
R
F
R
S
C
L
4
5
V
OUT
75
75
75
+3V OR +5V
RGB
MONITOR #1
0.1 F
10 F
AD8044
2
3
A
CL
(V/V)
6
Figure 35. Capacitive Load Drive vs. Closed-Loop Gain
APPLICATIONS
RGB Buffer
A
V+
75
1k
1k
The AD8044 can provide buffering of RGB signals that include
ground while operating from a single +3 V or +5 V supply.
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.
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.
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
that may use either a standalone triple DAC or DACs that are
integrated into a larger graphics chip.
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
W
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
W
to ground. In either
case, the monitor terminates its end of the line with 75
W.
The circuit in 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.
B
AD8044
75
75
75
1k
1k
75
75
AD8044
C
V–
1k
RGB
MONITOR #2
1k
Figure 36. Single Supply RGB Video Driver
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.
500mV
V
IN
100
90
5 s
GND
V
OUT
10
0%
GND
500mV
Figure 37. +3 V, RGB Buffer
–12–
REV. B
AD [ ANALOG DEVICES ]
