LMC6044 Datasheet, PDF (10/23 Pages) National Semiconductor (TI) – CMOS Quad Micropower Operational Amplifier

LMC6044
SNOS612D – NOVEMBER 1994 – REVISED MARCH 2013
APPLICATION HINTS
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AMPLIFIER TOPOLOGY
The LMC6044 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 outupt buffer stage, the output stage
is taken directly from the internal integrator, which provides both low output impedance and large gain. Special
feed-forward compensation design techniques are incorporated to maintain stability over a wider range of
operating conditions than traditional micropower op-amps. These features make the LMC6044 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 resistance with amplifiers with ultra-low input current, like the
LMC6044.
Although the LMC6044 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, photodiodes, and circuits board parasitics, reduce
phase margins.
When high input impedance are demanded, guarding of the LMC6044 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.)
Figure 27. Canceling the Effect of Input Capacitance
The effect of input capacitance can be compensated for by adding a capacitor. Adding a capacitor, C_f, around
the feedback resistor (as in Figure 27) such that:
(1)
or
R₁ C_IN ≤ R₂ C_f
(2)
Since it is often difficult to know the exact value of C_IN, C_f can be experimentally adjusted so that the desired
pulse response is achieved. Refer to the LMC660 and the LMC662 for a more detailed discussion on
compensating for input capacitance.
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 capacitive 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 external components, op amps can easily indirectly drive capacitive loads, as shown in Figure 28.
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