LMC6041 Datasheet, PDF (7/12 Pages) National Semiconductor (TI) – CMOS Single Micropower Operational Amplifier

Typical Performance Characteristics V_S = ± 7.5V, T_A = 25˚C unless otherwise
specified (Continued)
Stability vs
Capacitive Load
(A_V = ±10)
DS011136-43
Applications Hints
AMPLIFIER TOPOLOGY
The LMC6041 incorporates a novel op-amp design topology
that enables it to maintain rail-to-rail output swing even when
driving a large load. Instead of relying on a push-pull unity
gain output buffer stage, the output stage is taken directly
from the internal integrator, which provides both low output
impedance and large gain. Special feed-forward compensa-
tion design techniques are incorporated to maintain stability
over a wider range of operating conditions than traditional
micropower op-amps. These features make the LMC6041
both easier to design with, and provide higher speed than
products typically found in this ultra-low power class.
COMPENSATING FOR INPUT CAPACITANCE
It is quite common to use large values of feedback resis-
tance with amplifiers with ultra-low input current, like the
LMC6041.
Although the LMC6041 is highly stable over a wide range of
operating conditions, certain precautions must be met to
achieve the desired pulse response when a large feedback
resistor is used. Large feedback resistors and even small
values of input capacitance, due to transducers, photo-
diodes, and circuits board parasitics, reduce phase margins.
When high input impedance are demanded, guarding of the
LMC6041 is suggested. Guarding input lines will not only re-
duce leakage, but lowers stray input capacitance as well.
(See Printed-Circuit-Board Layout for High Impedance
Work.)
The effect of input capacitance can be compensated for by
adding a capacitor. Adding a capacitor, C_f, around the feed-
back resistor (as in Figure 1 ) such that:
or
R₁ C_IN ≤ R₂ C_f
Since it is often difficult to know the exact value of C_IN, C_f can
be experimentally adjusted so that the desired pulse re-
sponse is achieved. Refer to the LMC660 and the LMC662
for a more detailed discussion on compensating for input ca-
pacitance.
CAPACITIVE LOAD TOLERANCE
Direct capacitive loading will reduce the phase margin of
many op-amps. A pole in the feedback loop is created by the
combination of the op-amp’s output impedance and the ca-
pacitive load. This pole induces phase lag at the unity-gain
crossover frequency of the amplifier resulting in either an os-
cillatory or underdamped pulse response. With a few exter-
nal components, op amps can easily indirectly drive capaci-
tive loads, as shown in Figure 2.
DS011136-5
FIGURE 1. Cancelling the Effect of Input Capacitance
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