AD8180/AD8182

–8–

REV. B

APPLICATIONS

Multiplexing two RGB Video Sources

A common video application requires two RGB sources to be

multiplexed together before the selected signal is applied to a

monitor. Typically one source would be the PC’s normal output,

the second source might be a specialized source such as MPEG

video. Figure 23 shows how such a circuit could be realized

using the AD8180 and AD8182 and three current feedback op

amps. The video inputs to the multiplexers are terminated with

75

Ω resistors. This has the effect of halving the signal amplitude

of the applied signals.

Because all three multiplexers are permanently active, the

ENABLE pins are tied permanently low. The three SELECT

pins are tied together and this signal is used to select the source.

In order to drive a 75

Ω back terminated load (R

Ω), the

multiplexer outputs are buffered using the AD8001 current feed-

back op amp. A gain of two compensates for the signal halving by

the AD8001 output back termination resistor so that the system

has an overall gain of unity.

If lower speed and crosstalk can be tolerated, either of the triple

op amps, AD8013 or AD8073, can replace the three AD8001 op

amps in the above circuit. Because both devices have bandwidths

in the 100 MHz to 140 MHz range at a gain of +2, these ampli-

fiers will dominate the frequency response of the circuit.

With no signal present, the total quiescent current of the cir-

cuit in Figure 23 is 25.6 mA (3.8 mA + 6.8 mA + 3

× 5 mA), or

about 8.5 mA per channel. If either the AD8013 or AD8073

are used, the quiescent current will decrease to about 6.5 mA

per channel.

To reduce power consumption further, three AD8011 single

op amps can be used. With a quiescent current of 1 mA, this

will reduce the per channel quiescent current to about 4.5 mA.

Table II. Amplifier Options for RGB Multiplexer

Op Amp Comments

AD8001

AD8011

Lower Power Consumption, Bandwidth (G = +2) =

AD8013

Triple Op Amp, Bandwidth (G = +2) = 140 MHz,

AD8073

Lower Power Triple Op Amp, Bandwidth (G = +2) =

8

7

6

5

ENABLE

ENABLE A

ENABLE B

0.1 F

10 F

0.1 F

10 F

0.1 F

+

75

75

75

75

75

75

RG

B

COMPUTER

GRAPHICS

RG

B

0.1 F

10 F

75

681

10 F

0.1 F

0.1 F

10 F

75

681

681

10 F

0.1 F

0.1 F

10 F

75

681

10 F

0.1 F

SELECT

MONITOR

1

2

3

4

AD8180

+1

+1

1

2

3

4

14

13

12

11

AD8182

5

6

7

10

9

8

DECODER

+1

+1

DECODER

+1

+1

+

+

MPEG

AD8001

681

AD8001

681

AD8001

R

G

B

0.1 F

10 F

+

10 F

DECODER

Figure 23. Multiplexing Two Component Video Sources