LMC6572, LMC6574

SNOS707D – DECEMBER 1996 – REVISED MARCH 2013

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APPLICATIONS HINTS

LOW VOLTAGE AMPLIFIER TOPOLOGY

The LMC6574/2 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 LMC6574/2 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 for amplifiers with ultra-low input current, like the

LMC6574/2.

Although the LMC6574/2 is highly stable over a wide range of operating conditions, a large feedback resistor will

react even with small values of capacitance at the input of the op-amp to reduce phase margin. The capacitance

at the input of the op-amp comes from transducers, photodiodes and circuit board parasitics.

(as in Figure 30) such that:

(1)

or

(2)

pulse response is achieved. Refer to the LMC660 and LMC662 for a more detailed discussion on compensating

for input capacitance.

When high input impedances are demanded, guarding of the LMC6574/2 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 30. Cancelling the Effect of 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 31.

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