LMC6484IM datasheet(14/24 Pages) TI

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Application Information (Continued)

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

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

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

insulator. 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

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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