Note that these capacitor values are usually significant

smaller than those given by the older, more conservative

formula:

Using the smaller capacitors will give much higher band-

width with little degradation of transient response. It may be

necessary in any of the above cases to use a somewhat

larger feedback capacitor to allow for unexpected stray ca-

pacitance, or to tolerate additional phase shifts in the loop, or

excessive capacitive load, or to decrease the noise or band-

width, or simply because the particular circuit implementa-

tion needs more feedback capacitance to be sufficiently

stable. For example, a printed circuit board’s stray capaci-

tance may be larger or smaller than the breadboard’s, so the

actual optimum value for C

estimated using the breadboard. In most cases, the values

of C

the computed value.

CAPACITIVE LOAD TOLERANCE

Like many other op amps, the LMC660 may oscillate when

its applied load appears capacitive. The threshold of oscilla-

tion varies both with load and circuit gain. The configuration

most sensitive to oscillation is a unity-gain follower. See

Typical Performance Characteristics.

The load capacitance interacts with the op amp’s output

resistance to create an additional pole. If this pole frequency

is sufficiently low, it will degrade the op amp’s phase margin

so that the amplifier is no longer stable at low gains. As

shown in Figure 3, the addition of a small resistor (50

Ω to

100

Ω) in series with the op amp’s output, and a capacitor (5

pF to 10 pF) from inverting input to output pins, returns the

phase margin to a safe value without interfering with lower-

frequency circuit operation. Thus larger values of capaci-

tance can be tolerated without oscillation. Note that in all

cases, the output will ring heavily when the load capacitance

is near the threshold for oscillation.

Capacitive load driving capability is enhanced by using a pull

up resistor to V

+ (Figure 4). Typically a pull up resistor

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

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 LMC662, typically less

than 0.04 pA, it is essential to have an excellent layout.

Fortunately, the techniques for 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 acceptably 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 LMC660’s inputs and the

terminals of capacitors, diodes, conductors, resistors, relay

terminals, etc. connected to the op amp’s inputs. SeeFigure

5. 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 an input. This

would cause a 100 times degradation from the LMC660’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

00876706

FIGURE 2. General Operational Amplifier Circuit

00876705

FIGURE 3. Rx, Cx Improve Capacitive Load Tolerance

00876723

FIGURE 4. Compensating for Large Capacitive Loads

with a Pull Up Resistor

www.national.com

8