LMC7111_14 Datasheet, PDF (16/26 Pages) National Semiconductor (TI) – LMC7111 Tiny CMOS Operational Amplifier with Rail-to-Rail Input and Output

LMC7111
SNOS753E – AUGUST 1999 – REVISED MARCH 2013
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CAPACITIVE LOAD TOLERANCE
The LMC7111 can typically directly drive a 300 pF load with V_S = 10V at unity gain without oscillating. The unity
gain follower is the most sensitive configuration. Direct capacitive loading reduces the phase margin of op-amps.
The combination of the op-amp's output impedance and the capacitive load induces phase lag. This results in
either an underdamped pulse response or oscillation.
Capacitive load compensation can be accomplished using resistive isolation as shown in Figure 53. This simple
technique is useful for isolating the capacitive input of multiplexers and A/D converters.
Figure 53. Resistive Isolation
of a 330 pF Capacitive Load
COMPENSATING FOR INPUT CAPACITANCE WHEN USING LARGE VALUE FEEDBACK
RESISTORS
When using very large value feedback resistors, (usually > 500 kΩ) the large feed back resistance can react with
the input capacitance due to transducers, photodiodes, and circuit board parasitics to reduce phase margins.
The effect of input capacitance can be compensated for by adding a feedback capacitor. The feedback capacitor
(as in Figure 54), C_f is first estimated by:
(1)
or
R₁ C_IN ≤ R₂ C_f
(2)
which typically provides significant overcompensation.
Printed circuit board stray capacitance may be larger or smaller than that of a breadboard, so the actual optimum
value for C_F may be different. The values of C_F should be checked on the actual circuit. (Refer to the LMC660
quad CMOS amplifier data sheet for a more detailed discussion.)
Figure 54. Cancelling the Effect of Input Capacitance
OUTPUT SWING
The output of the LMC7111 will go to within 100 mV of either power supply rail for a 10 kΩ load and to 20 mV of
the rail for a 100 kΩ load. This makes the LMC7111 useful for driving transistors which are connected to the
same power supply. By going very close to the supply, the LMC7111 can turn the transistors all the way on or all
the way off.
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