LMC6064IM datasheet(7/12 Pages) NSC

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Typical Performance Characteristics (Continued)

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AMPLIFIER TOPOLOGY

The LMC6064 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 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 LMC6064

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 for amplifiers with ultra-low input current, like the

LMC6064.

Although the LMC6064 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 circuit board parasitics, reduce phase margins.

When high input impedances are demanded, guarding of the

LMC6064 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. Place a capacitor, C

f, around the feed-

back resistor (as in

Figure 1 ) such that:

1 CIN ≤ R2 Cf

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

IN,Cf can

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

capacitance.

Non-Inverting Large

Signal Pulse Response

DS011466-33

Crosstalk Rejection

vs Frequency

DS011466-34

Stability vs Capacitive

Load, R

L = 20 kΩ

DS011466-35

Stability vs Capacitive

Load R

L = 1MΩ

DS011466-36

DS011466-4

FIGURE 1. Canceling the Effect of Input Capacitance

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