Non-Inverting Large

Signal Pulse Response

Crosstalk Rejection

vs Frequency

01146735

01146736

Stability vs Capacitive

Load, R

Ω

Stability vs Capacitive

Load R

Ω

01146737

01146738

Applications Hints

AMPLIFIER TOPOLOGY

The LMC6084 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 LMC6084

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

LMC6084.

Although the LMC6084 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

LMC6084 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).

The effect of input capacitance can be compensated for by

adding a capacitor, C

Figure 1 ) such that:

or

R

≤ 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 LMC662 for a

more detailed discussion on compensating for input capaci-

tance.

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