AD8074/AD8075
Triple Video Multiplexer
and the amplifier output are disabled to a high impedance to
provide a high-impedance disabled state.
The AD8074 and AD8075 each have an output-enable function
that can be used to disable the outputs and put them in a high-
impedance state. Usually, for a unity-gain device, it is relatively
easy to provide high disabled impedance, because the feedback
path is from the output to a high-impedance input. However, for a
non-unity-gain part, the feedback provides a resistive path to
ground. This will usually dominate the disabled output imped-
ance, and make it a much lower value than the unity-gain device.
To construct a multiplexer, the outputs from one or more devices
are connected in parallel and only one device is enabled at a
time while all of the others are disabled. The two sets of inputs
are applied individually to each of the separate device inputs.
Figure 4 shows the circuit details for this function. The first RGB
Source 1 is input to the first AD8075. Each of the individual
signals is terminated to ground with 75 Ω to provide proper
termination for the input cables. In a similar fashion, the Source
2 signals are input to the second AD8075.
The AD8075 has an internal buffer that provides a low-impedance,
ground level output that terminates the feedback path during
enabled operation. In the disabled state, both this buffer output
+5V
+5V
+5V
+
25F
0.1F
0.1F
0.1F
75⍀
R
R
G
B
75⍀
75⍀
75⍀
AD8075
75⍀
75⍀
SOURCE 1
G
B
OUTPUT
OE
25F
0.1F
0.1F
0.1F
+
–5V
–5V
–5V
SEL1/SEL2
+5V
+5V
+5V
+
25F
0.1F
0.1F
0.1F
OE
75⍀
75⍀
75⍀
R
G
B
75⍀
75⍀
75⍀
AD8075
SOURCE 2
25F
0.1F
0.1F
0.1F
+
–5V
–5V
–5V
Figure 4. Mux
–11–
REV. A
ROCHESTER [ Rochester Electronics ]
