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

ducting 10 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 LMC6044, typically less

than 2 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 LMC6044’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 amplifer 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 LMC6044’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.

The designer should be aware that when it is inappropriate

to lay out a PC board for the sake of just a few circuits, there

is another technique which is even better than a guard ring

on a PC board: Don’t insert the amplifier’s input pin into the

board at all, but bend it up in the air and use only air as an in-

sulator. Air is an excellent insulator. In this case you may

have to forego some of the advantages of PC board con-

struction, but the advantages are sometimes well worth the

effort

of

using

point-to-point

up-in-the-air

wiring.

See

Figure 6.

DS011138-18

FIGURE 3. Compensating for Large

Capacitive Loads with a Pull Up Resistor

DS011138-7

FIGURE 4. Example of Guard Ring

in P.C. Board Layout

DS011138-8

Inverting Amplifier

DS011138-10

Non-Inverting Amplifier

DS011138-9

Follower

FIGURE 5. Typical Connections of Guard Rings

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