REV. B
AD8041
–13–
APPLICATIONS
RGB Buffer
The AD8041 can provide buffering of RGB signals that include
ground while operating from a single 3 V or 5 V supply.
The signals that drive an RGB monitor are usually supplied by
current output DACs that operate from a 5 V only supply. These
can triple DACs like the ADV7120 and ADV7122 from Analog
Devices or integrate into the graphics controller IC as in most
PCs these days.
During the horizontal blanking interval, the currents output
from the DACs go to zero and the RGB signals are pulled to
ground via the termination resistors. If more than one RGB
monitor is desired, it cannot simply be connected in parallel
because it will provide an additional termination. Therefore,
buffering must be provided before connecting a second monitor.
Since the RGB signals include ground as part of their dynamic
output range, it has previously been required to use a dual-
supply op amp to provide this buffering. In some systems, this is
the only component that requires a negative supply, so it can be
quite inconvenient to incorporate this multiple monitor feature.
Figure 11 shows a schematic of one channel of a single-supply,
gain-of-two buffer for driving a second RGB monitor. No cur-
rent is required when the amplifier output is at ground. The
termination resistor at the monitor helps pull the output down
at low voltage levels.
6
4
7
3
2
AD8041
1k⍀
75⍀
10F
8
75⍀
R, G OR B
NC
0.1F
1k⍀
PRIMARY RGB
MONITOR
75⍀
SECOND RGB
MONITOR
3V OR 5V
Figure 11. Single-Supply RGB Buffer
Figure 12 is an oscilloscope photo of the circuit in Figure 11
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 AD8041 is 1.4 V with the termination resistors providing a
divide-by-two. The red and green signals can be buffered in the
same manner with duplication of this circuit.
V
IN
GND
GND
V
OUT
10
0%
100
90
5
s500mV
500mV
Figure 12. 3 V, RGB Buffer
Single-Supply Composite Video Line Driver
Figure 13 shows a schematic of a single-supply gain-of-two
composite video line driver. Since the sync tips of a composite
video signal extend below ground, the input must be ac-coupled
and shifted positively to provide signal swing during these nega-
tive excursions in a single-supply configuration.
The input is terminated in 75 Ω and ac-coupled via C
IN
to a
voltage divider that provides the dc bias point to the input.
Setting the optimal bias point requires some understanding of
the nature of composite video signals and the video performance
of the AD8041.
Signals of bounded peak-to-peak amplitude that vary in duty
cycle require larger dynamic swing capability than their peak-to-
peak amplitude after ac coupling. As a worst case, the dynamic
signal swing required will approach twice the peak-to-peak value.
The two bounding cases are for a duty cycle that is mostly low,
but occasionally goes high at a fraction of a percent duty cycle
and vice versa.
Composite video is not quite this demanding. One bounding
extreme is for a signal that is mostly black for an entire frame
but has a white (full intensity), minimum width spike at least
once per frame.
The other extreme is for a video signal that is full white every-
where. The blanking intervals and sync tips of such a signal will
have negative going excursions in compliance with composite
video specifications. The combination of horizontal and vertical
blanking intervals limit such a signal to being at its highest level
(white) for only about 75% of the time.
As a result of the duty cycle variations between the two extremes
presented above, a 1 V p-p composite video signal that is multi-
plied by a gain of two requires about 3.2 V p-p of dynamic voltage
swing at the output for an op amp to pass a composite video
signal of arbitrary duty cycle without distortion.
Some circuits use a sync tip clamp along with ac coupling to
hold the sync tips at a relatively constant level in order to lower
the amount of dynamic signal swing required. However, these
circuits can have artifacts like sync tip compression unless they
are driven by sources with very low output impedance.
6
4
7
3
2
AD8041
R
F
1k⍀
10k⍀
10F
5V
75⍀
COMPOSITE
VIDEO IN
NC
0.1F
R
T
75⍀
8
1000F
0.1F
4.99k⍀
10F
4.99k⍀
47F
R
G
1k⍀
220F
75⍀
COAX
R
L
75⍀
V
OUT
Figure 13. Single-Supply Composite Video Line Driver
The AD8041 not only has ample signal swing capability to
handle the dynamic range required without using a sync tip
clamp but also has good video specifications like differential
gain and differential phase when buffering these signals in an ac-
coupled configuration.
