REV. A

AD8074/AD8075

–11–

Triple Video Multiplexer

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.

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

and the amplifier output are disabled to a high impedance to

provide a high-impedance disabled state.

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.

0.1 F

–5V

0.1 F

–5V

0.1 F

–5V

+

25 F

0.1 F

+5V

0.1 F

+5V

0.1 F

+5V

+

25 F

AD8075

75

75

75

OE

75

75

75

R

G

B

SOURCE 2

0.1 F

–5V

0.1 F

–5V

0.1 F

–5V

+

25 F

0.1 F

+5V

0.1 F

+5V

0.1 F

+5V

+

25 F

AD8075

75

75

75

OE

75

75

75

R

G

B

SOURCE 1

SEL1/

SEL2

R

G

B

OUTPUT

Figure 4. Mux