MAX4012 Maxim, MAX4012 Datasheet - Page 12

• Manufacturer Part Number: MAX4012
• Description: The MAX4012 single, MAX4016 dual, MAX4018 triple, and MAX4020 quad op amps are unity-gain-stable devices that combine high-speed performance with Rail-to-Rail outputs
• Manufacturer: Maxim

Low-Cost, High-Speed, Single-Supply
Op Amps with Rail-to-Rail Outputs
Figure 2. Enable Logic-Low Input Current vs. V
Figure 4. Enable Logic-Low Input Current vs. V
Series Resistor
12
______________________________________________________________________________________
-100
-120
-140
-160
-10
-20
-40
-60
-80
-1
-2
-3
-4
-5
-6
-7
-8
-9
20
0
0
0
0
50 100 150
50 100 150
mV ABOVE V
mV ABOVE V
200 250
200 250
300 350
300 350
EE
EE
400 450
400 450
IL
IL
with 10k
500
500
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
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
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
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
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.
Figure 3. Circuit to Reduce Enable Logic-Low Input Current
S
) vs. capacitive load. Figure 7 shows how a capaci-
Output Capacitive Loading and Stability
to 30 ) placed before the reactive load
IN-
IN+
MAX40_ _
ENABLE
EN_
10k
isolation resistor on
OUT