LMC6042_17 Datasheet, PDF (10/24 Pages) Texas Instruments – CMOS Dual Micropower Operational Amplifier

LMC6042
SNOS611E – AUGUST 1999 – REVISED MARCH 2013
APPLICATIONS HINTS
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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 compensation 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 resistance 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, photodiodes, 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).
Figure 29. Cancelling the Effect of Input Capacitance
The effect of input capacitance can be compensated for by adding a capacitor. Place a capacitor, C_f, around the
feedback resistor (as in Figure 29 ) 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 30.
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