of the ultra-low input current of the LMC6484, typically less

than 20 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 LMC6484’s inputs and the

terminals of capacitors, diodes, conductors, resistors, relay

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

Fig-

ure 9. To have a significant effect, guard rings should be

placed in 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 250 times degradation from the LMC6484’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 10 for

typical 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 11.

DS011714-20

FIGURE 9. Example of Guard Ring in P.C. Board

Layout

DS011714-21

Inverting Amplifier

DS011714-22

Non-Inverting Amplifier

DS011714-23

Follower

FIGURE 10. Typical Connections of Guard Rings

DS011714-24

(Input pins are lifted out of PC board and soldered directly to components.

All other pins connected to PC board.)

FIGURE 11. Air Wiring

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