LMC6001 Datasheet, PDF (15/25 Pages) National Semiconductor (TI) – Ultra Ultra-Low Input Current Amplifier

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8 Applications and Implementation
LMC6001
SNOS694I – MARCH 1995 – REVISED SEPTEMBER 2015
NOTE
Information in the following applications sections is not part of the TI component
specification, and TI does not warrant its accuracy or completeness. TI’s customers are
responsible for determining suitability of components for their purposes. Customers should
validate and test their design implementation to confirm system functionality.
8.1 Application Information
8.1.1 Compensating For Input Capacitance
It is quite common to use large values of feedback resistance for amplifiers with ultra-low input current, like the
LMC6001.
Although the LMC6001 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 with
even small values of input capacitance, due to transducers, photodiodes, and printed-circuit-board parasitics,
reduce phase margins.
When high input impedances are demanded, TI suggests guarding the LMC6001. 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_f, around the feedback resistors
(as in Figure 19) such that:
(1)
or
R₁ C_IN ≤ R₂ C_f
(2)
Because 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 (SNOSBZ3) and LMC662 (SNOSC51) for a more detailed
discussion on compensating for input capacitance.
Figure 19. Cancelling the Effect of Input Capacitance
8.1.2 Capacitive Load Tolerance
All rail-to-rail output swing operational amplifiers have voltage gain in the output stage. A compensation capacitor
is normally included in this integrator stage. The frequency location of the dominant pole is affected by the
resistive load on the amplifier. Capacitive load driving capability can be optimized by using an appropriate
resistive load in parallel with the capacitive load. See Typical Characteristics.
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 output impedance of the op amp 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 20.
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