AD8074/AD8075
RGB Buffer for Second Monitor
can be connected to the same signal, as is done in some studio-
type TV monitors.
The RGB signals for PC monitors are driven through coax
cables whose characteristic impedance is 75 Ω. The graphics
chip will generally have current-source output drivers that should
be double terminated with a 75 Ω shunt termination at each end.
On the transmit end, the shunt terminations are provided to
ground close to the graphics IC, while the monitor terminates
its end via internal termination resistors. While this scheme works
well and is virtually foolproof for a single monitor, it leaves no
means for passively connecting a second monitor to the same source.
A way around this problem is to connect the first monitor to the
RGB channels in the standard fashion, and then to provide a
triple gain-of-two buffer to drive the second monitor. The AD8075
is designed to provide this function and also provide excellent
high-frequency performance for high-resolution graphics signals.
Figure 3 shows a schematic of this circuit.
The outputs of the AD8075 are low impedance voltage sources
and are therefore series-terminated with 75 Ω resistors. The
internal resistors in Monitor #2 provide the terminations at its
end. The overall effect of this type of termination scheme is to
divide the signal amplitude by two. This is compensated by the
gain of two provided by the AD8075.
A second monitor that is connected simply in parallel will pro-
vide an extra set of terminations that will upset the signal levels.
To keep costs low, most computer monitors do not have the ability
to open-circuit the terminations in order that an additional monitor
MONITOR #1
PC GRAPHICS IC
R
75⍀
75⍀
CURRENT SOURCE
OUTPUT DRIVERS
G
INTERNAL
75⍀
75⍀
TERMINATIONS
B
75⍀
75⍀
+5V
+5V
+5V
+
25F
0.1F
0.1F
0.1F
MONITOR #2
75⍀
75⍀
75⍀
75⍀
AD8075
INTERNAL
TERMINATIONS
75⍀
75⍀
25F
0.1F
0.1F
0.1F
+
–5V
–5V
–5V
Figure 3. Buffer
–10–
REV. A
ROCHESTER [ Rochester Electronics ]
