LMC6084AIMX NSC [National Semiconductor], LMC6084AIMX Datasheet - Page 8

• Manufacturer Part Number: LMC6084AIMX
• Description: Precision CMOS Quad Operational Amplifier
• Manufacturer: NSC [National Semiconductor]

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Typical Performance Characteristics
Applications Hints
AMPLIFIER TOPOLOGY
The LMC6084 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 LMC6084
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
LMC6084.
Although the LMC6084 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.
Signal Pulse Response
Stability vs Capacitive
Non-Inverting Large
Load, R
L
= 600Ω
01146735
01146737
8
(Continued)
When high input impedances are demanded, guarding of the
LMC6084 is suggested. Guarding input lines will not only
reduce 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, C
Figure 1 ) such that:
Since it is often difficult to know the exact value of C
be experimentally adjusted so that the desired pulse re-
sponse is achieved. Refer to the LMC660 and LMC662 for a
more detailed discussion on compensating for input capaci-
tance.
Stability vs Capacitive
Crosstalk Rejection
Load R
vs Frequency
f
, around the feedback resistors (as in
R
1
C
IN
L
or
= 1 MΩ
≤ R
2
C
f
01146738
01146736
IN
, C
f
can