Datasheet

MAX4012/MAX4016/MAX4018/MAX4020
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
(R
S
) vs. capacitive load. Figure 7 shows how a capaci-
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 20Ω 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 27Ω 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, Single-Supply
Op Amps with Rail-to-Rail Outputs
12 ______________________________________________________________________________________
OUT
IN-
EN_
IN+
10kΩ
ENABLE
MAX40_ _
20
-160
050100 150 300 350 500
-100
-120
0
mV ABOVE V
EE
INPUT CURRENT (µA)
200 250 400 450
-60
-140
-20
-40
-80
0
-10
050100 150 300 350 500
-7
-8
-1
mV ABOVE V
EE
INPUT CURRENT (µA)
200 250 400 450
-3
-5
-9
-2
-4
-6
Figure 2. Enable Logic-Low Input Current vs. V
IL
Figure 4. Enable Logic-Low Input Current vs. V
IL
with 10kΩ
Series Resistor
Figure 3. Circuit to Reduce Enable Logic-Low Input Current