Application Hints

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 compensa-

tion 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 resis-

tance 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, photo-

diodes, 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 re-

duce 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. Adding a capacitor, C

back resistor (as in

Figure 1 ) such that:

or

R

Since it is often difficult to know the exact value of C

be experimentally adjusted so that the desired pulse re-

sponse is achieved. Refer to the LMC660 and the LMC662

for a more detailed discussion on compensating for input ca-

pacitance.

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 ca-

pacitive load. This pole induces phase lag at the unity-gain

crossover frequency of the amplifier resulting in either an os-

cillatory or underdamped pulse response. With a few exter-

nal components, op amps can easily indirectly drive capaci-

tive loads, as shown in

Figure 2.

In the circuit of

Figure 2, R1 and C1 serve to counteract the

loss of phase margin by feeding the high frequency compo-

Stability vs Capacitive Load

DS011138-43

Stability vs Capacitive Load

DS011138-44

DS011138-5

FIGURE 1. Canceling the Effect of Input Capacitance

DS011138-6

FIGURE 2. LMC6044 Noninverting Gain of 10 Amplifier,

Compensated to Handle Capacitive Loads

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