AD810

REV. A

–15–

N:1 MULTIPLEXER

A multiplexer of arbitrary size can be formed by combining the

desired number of AD810s together with the appropriate

selection logic. The schematic in Figure 58 shows a

recommendation for a 4:1 mux which may be useful for driving

a high impedance such as the input to a video A/D converter

(such as the AD773). The output series resistors effectively

compensate for the combined output capacitance of the OFF

channels plus the input capacitance of the A/D while

maintaining wide bandwidth. In the case illustrated, the –0.1 dB

bandwidth is about 20 MHz with no peaking. Switching time

and OFF channel isolation (for the 4:1 mux) are about 250 ns

and 60 dB at 10 MHz, respectively.

–0.5

–3.0

1

10

100

–1.0

–1.5

–2.0

–2.5

0

0.5

0

–45

–90

–135

–180

–225

PHASE

GAIN

FREQUENCY – MHz

Figure 56. 4:1 Mux ON Channel Gain and Phase vs.

Frequency

–50

1

10

100

–60

–70

–40

–30

FREQUENCY – MHz

Figure 57. 4:1 Mux OFF Channel Feedthrough vs.

Frequency

AD810

75

Ω

7

2

3

0.1µF

6

1k

Ω

4

0.1µF

8

SELECT A

AD810

7

2

3

0.1µF

6

4

0.1µF

8

SELECT D

AD810

7

2

3

0.1µF

6

4

0.1µF

8

SELECT C

AD810

7

2

3

0.1µF

6

4

0.1µF

8

SELECT B

33

Ω

75

Ω

1k

Ω

33

Ω

75

Ω

1k

Ω

33

Ω

75

Ω

1k

Ω

33

Ω

Figure 58. A 4:1 Multiplexer Driving a High Impedance