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

conducting 500 µA or more will significantly improve capaci-

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

determined based on the current sinking capability of the

amplifier with respect to the desired output swing. Open loop

gain of the amplifier can also be affected by the pull up resis-

tor (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 LMC6082, 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 LMC6082’s inputs and the

terminals of capacitors, diodes, conductors, resistors, relay

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

Fig-

ure 4. To have a significant effect, guard rings should be

placed on both the top and bottom of the PC board. This PC

foil must then be connected to a voltage which is at the same

voltage as the amplifier inputs, since no leakage current can

flow between two points at the same potential. For example,

a PC board trace-to-pad resistance of 10

12

Ω, which is nor-

mally considered a very large resistance, could leak 5 pA if

the trace were a 5V bus adjacent to the pad of the input. This

would cause a 100 times degradation from the LMC6082’s

actual performance. However, if a guard ring is held within

5 mV of the inputs, then even a resistance of 10

11

Ω would

cause only 0.05 pA of leakage current. See

Figure 5 for typi-

cal connections of guard rings for standard op-amp

configurations.

DS011297-4

FIGURE 1. Cancelling the Effect of Input Capacitance

DS011297-5

FIGURE 2. LMC6082 Noninverting Gain of 10 Amplifier,

Compensated to Handle Capacitive Loads

DS011297-14

FIGURE 3. Compensating for Large Capacitive Loads

with a Pull Up Resistor

DS011297-6

FIGURE 4. Example of Guard Ring in P.C. Board

Layout

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