To implement the mux function, the outputs of multiple

amplifiers can be tied together, and only the amplifier

with the selected input will be enabled. All of the other

amplifiers will be placed in the low-power shutdown

mode, with their high output impedance presenting

very little load to the active amplifier output. For gains

of +2 or greater, the feedback network impedance of

all the amplifiers used in a mux application must be

considered when calculating the total load on the

active amplifier output

Output Capacitive Loading and Stability

The MAX4012/MAX4016/MAX4018/MAX4020 are opti-

mized for AC performance. They are not designed to

drive highly reactive loads, which decreases phase

margin and may produce excessive ringing and oscilla-

tion. Figure 5 shows a circuit that eliminates this prob-

lem. Figure 6 is a graph of the optimal isolation resistor

tive load causes excessive peaking of the amplifier’s

frequency response if the capacitor is not isolated from

the amplifier by a resistor. A small isolation resistor

(usually 20

Ω to 30Ω) placed before the reactive load

prevents ringing and oscillation. At higher capacitive

loads, AC performance is controlled by the interaction

of the load capacitance and the isolation resistor.

Figure 8 shows the effect of a 27

Ω isolation resistor on

closed-loop response.

Coaxial cable and other transmission lines are easily

driven when properly terminated at both ends with their

characteristic impedance. Driving back-terminated

transmission lines essentially eliminates the line’s

capacitance.

Low-Cost, High-Speed, SOT23, Single-Supply

Op Amps with Rail-to-Rail Outputs

12

______________________________________________________________________________________

OUT

IN-

EN_

IN+

10k

ENABLE

MAX40_ _

20

-160

0

50 100 150

300 350

500

-100

-120

0

200 250

400 450

-60

-140

-20

-40

-80

0

-10

0

50 100 150

300 350

500

-7

-8

-1

200 250

400 450

-3

-5

-9

-2

-4

-6

Series Resistor

Figure 3. Circuit to Reduce Enable Logic-Low Input Current