AD8184

–8–

REV. 0

Chip capacitors should be used for supply bypassing. One end

of the capacitor should be connected to the ground plane and

the other within 1/4 inch of each power pin. An additional large

(4.7

µF–10 µF) tantalum capacitor should be connected in par-

allel with each of the smaller capacitors for low impedance sup-

ply bypassing over a broad range of frequencies.

Signal traces should be as short as possible. Stripline or

microstrip techniques should be used for long (longer than

about 1 inch) signal traces. These should be designed with a

characteristic impedance of 50

Ω or 75 Ω and be properly ter-

minated at each end using surface mount components.

Careful layout is imperative to minimize crosstalk. Guards

(ground or supply traces) must be run between all signal traces

to limit direct capacitive coupling. Input and output signal lines

should fan out away from the mux as much as possible. If mul-

tiple signal layers are available, a buried stripline structure hav-

ing ground plane above, below and between signal traces will

have the best crosstalk performance.

Return currents flowing through termination resistors can also

increase crosstalk if these currents flow in sections of the finite-

impedance ground circuit shared between more than one input

or output. Minimizing the inductance and resistance of the ground

plane can reduce this effect, but further care should be taken in po-

sitioning the terminations. Terminating cables directly at the con-

nectors will minimize the return current flowing on the board, but

the signal trace between the connector and the mux will look like

an open stub and will degrade the frequency response. Moving the

termination resistors close to the input pins will improve the fre-

quency response, but the terminations from neighboring inputs

should not have a common ground return.

APPLICATIONS

A Buffered 4-to-1 Multiplexer

In applications where the output of a multiplexer must drive a

back-terminated 75

Ω line (R

to buffer the output of the AD8184. In the example in Figure

22, this is accomplished using the AD8009 high speed current

feedback op amp. The amplifier is configured with a gain of 2 to

compensate for the signal halving due to termination at the multi-

plexer input. This gives the overall circuit a gain of unity.

If lower speed can be tolerated, a number of other amplifiers

can replace the AD8009 op amp in the above circuit. In general

there is a trade-off between bandwidth and power consumption.

Table II summarizes the bandwidth and power consumption

characteristics of these op amps.

Table II. Amplifier Options for Multiplexer Buffering

Op Amp Comments

AD8009

14 mA

AD8001

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

AD8011

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

AD8079

Fixed Gain Dual Amplifier (2 or 2.2), Bandwidth =

AD8005

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

µA

681

Ω

+1

DECODER

1

2

3

4

5

6

7

14

13

12

11

10

9

8

+1

+1

+1

AD8184

75

Ω

75

Ω

75

Ω

75

Ω

NC

GND

GND

GND

0.1µF

10µF

0.1µF

10µF

10µF

0.1µF

10µF

0.1µF

A0

A1

75

Ω

681

Ω

AD8009

IN0

IN1

IN2

IN3

Figure 22. A Buffered 4-to-1 Multiplexer