nent of the output signal back to the amplifier’s inverting in-

put, thereby preserving phase margin in the overall feedback

loop.

4.0 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 LMC6574/2, 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 LMC6574/2’s inputs and

the terminals of capacitors, diodes, conductors, resistors, re-

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

Figure 3. 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 250 times degradation from the LMC6574/2’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 4 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

5.

DS011934-8

FIGURE 3. Example of Guard Ring in P.C. Board

Layout

DS011934-9

Inverting Amplifier

DS011934-10

Non-Inverting Amplifier

DS011934-11

Follower

FIGURE 4. Typical Connections of Guard Rings

DS011934-12

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

All other pins connected to PC board).

FIGURE 5. Air Wiring

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