LMC6081IN datasheet(7/12 Pages) NSC

Part #	LMC6081IN

Description	Precision CMOS Single Operational Amplifier

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Manufacturer	NSC [National Semiconductor (TI)]

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Applications Hints (Continued)

Although the LMC6081 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

LMC6081 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, C

f, around the feedback resistors (as in

Figure 1 ) such that:

or

R

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 LMC662 for a

more detailed discussion on compensating for input

capacitance.

CAPACITIVE LOAD TOLERANCE

All rail-to-rail output swing operational amplifiers have volt-

age gain in the output stage. A compensation capacitor is

normally included in this integrator stage. The frequency lo-

cation of the dominant pole is affected by the resistive load

on the amplifier. Capacitive load driving capability can be op-

timized 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 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-

nent of the output signal back to the amplifier’s inverting in-

put, thereby preserving phase margin in the overall feedback

loop.

Capacitive load driving capability is enhanced by using a pull

up resistor to V

+ (Figure 3). Typically a pull up resistor con-

ducting 500 µA or more will significantly improve capacitive

load responses. The value of the pull up resistor must be de-

termined based on the current sinking capability of the ampli-

fier with respect to the desired output swing. Open loop gain

of the amplifier can also be affected by the pull up resistor

(see electrical characteristics).

PRINTED-CIRCUIT-BOARD LAYOUT

FOR HIGH-IMPEDANCE WORK

It is generally recognized that any circuit which must operate

with less than 1000 pA of leakage current requires special

layout of the PC board. When one wishes to take advantage

of the ultra-low bias current of the LMC6081, typically less

than 10 fA, it is essential to have an excellent layout. Fortu-

nately, the techniques of obtaining low leakages are quite

simple. First, the user must not ignore the surface leakage of

the PC board, even though it may sometimes appear accept-

ably low, because under conditions of high humidity or dust

or contamination, the surface leakage will be appreciable.

To minimize the effect of any surface leakage, lay out a ring

of foil completely surrounding the LMC6081’s inputs and the

terminals of capacitors, diodes, conductors, resistors, relay

terminals, etc. connected to the op-amp’s inputs, as in

Fig-

DS011423-4

FIGURE 1. Cancelling the Effect of Input Capacitance

DS011423-5

FIGURE 2. LMC6081 Noninverting Gain of 10 Amplifier,

Compensated to Handle Capacitive Loads

DS011423-14

FIGURE 3. Compensating for Large

Capacitive Loads with a Pull Up Resistor

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