LMC6042AIN National Semiconductor, LMC6042AIN Datasheet - Page 9

• Manufacturer Part Number: LMC6042AIN
• Description: Operational Amplifier (Op-Amp) IC
• Manufacturer: National Semiconductor

Specifications of LMC6042AIN

• Op Amp Type: Low Power
• No. Of Amplifiers: 2
• Slew Rate: 0.02V/µs
• No. Of Pins: 8
• Operating Temperature Range: -40°C To +85°C
• I/o Type: Rail-Rail Outputs
• Mounting Type: Through Hole
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

Applications Hints
AMPLIFIER TOPOLOGY
The LMC6042 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 LMC6042
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 curent, like the
LMC6042.
Although the LMC6042 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
LMC6042 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. Place a capacitor, C
back resistor (as in Figure 1 ) such that:
or
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 the LMC662
for a more detailed discussion on compensating for input
capacitance.
FIGURE 1. Cancelling the Effect of Input Capacitance
R1 C
IN
≤ R2 C
f
f
, around the feed-
01113705
IN
, C
f
can
9
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
oscillatory or underdamped pulse response. With a few ex-
ternal components, op amps can easily indirectly drive ca-
pacitive loads, as shown in Figure 2.
In the circuit of Figure 2, R1 and C1 serve to counteract the
loss of phase margin by feeding the high frequency compo-
nent of the output signal back to the amplifier’s inverting
input, thereby preserving phase margin in the overall feed-
back loop.
Capacitive load driving capability is enhanced by using a
pull up resistor to V
conducting 10 µA or more will significantly improve capaci-
tive load responses. The value of the pull up resistor must be
determined based on the current sinking capability of the
amplifier with respect to the desired output swing. Open loop
gain of the amplifier can also be affected by the pull up
resistor (see Electrical Characteristics).
PRINTED-CIRCUIT-BOARD LAYOUT FOR
HIGH-IMPEDANCE WORK
It is generally recognized that any circuit which must operate
with less than 1000 pA of leakage current requires special
layout of the PC board. When one wishes to take advantage
of the ultra-low bias current of the LMC6042, typically less
than 2 fA, it is essential to have an excellent layout. Fortu-
nately, the techniques of obtaining low leakages are quite
FIGURE 2. LMC6042 Noninverting Gain of 10 Amplifier,
Compensated to Handle Capacitive Loads
Capacitive Loads with a Pull Up Resistor
FIGURE 3. Compensating for Large
+
(Figure 3). Typically a pull up resistor
01113718
01113706
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