LMC6061 Datasheet, PDF (6/12 Pages) National Semiconductor (TI) – Precision CMOS Single Micropower Operational Amplifier

Typical Performance Characteristics V_S = ±7.5V, T_A = 25˚C, Unless otherwise
specified (Continued)
Inverting Small Signal
Pulse Response
Inverting Large Signal
Pulse Response
Non-Inverting Small
Signal Pulse Response
DS011422-30
Non-Inverting Large
Signal Pulse Response
DS011422-31
Stability vs Capacitive
Load, R_L = 20 kΩ
DS011422-32
Stability vs Capacitive
Load R_L = 1 MΩ
DS011422-33
Applications Hints
AMPLIFIER TOPOLOGY
The LMC6061 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 LMC6061
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 for amplifiers with ultra-low input current, like the
LMC6061.
Although the LMC6061 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 circuit board parasitics, reduce phase margins.
When high input impedances are demanded, guarding of the
LMC6061 is suggested. Guarding input lines will not only re-
DS011422-34
DS011422-35
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. Place 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
capacitance.
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