LMC6082

SNOS630D – AUGUST 2000 – REVISED MARCH 2013

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APPLICATIONS HINTS

AMPLIFIER TOPOLOGY

The LMC6082 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 LMC6082 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

LMC6082.

Although the LMC6082 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 LMC6082 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)

(as in Figure 25 ) 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.

Figure 25. Cancelling the Effect of Input Capacitance

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 curves).

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 26.

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