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

tunately, 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 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 LMC6032’s inputs and the

terminals of capacitors, diodes, conductors, resistors, relay

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

Figure

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 LMC6032’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, or perhaps a minor

(2:1) degradation of the amplifier’s performance. See

Figure

6a, Figure 6b, Figure 6c for typical connections of guard

rings for standard op-amp configurations. If both inputs are

active and at high impedance, the guard can be tied to

ground and still provide some protection; see

Figure 6d.

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 7.

DS011135-6

FIGURE 5. Example of Guard Ring in

P.C. Board Layout

DS011135-7

(a) Inverting Amplifier

DS011135-8

(b) Non-Inverting Amplifier

DS011135-9

(c) Follower

DS011135-10

(d) Howland Current Pump

FIGURE 6. Guard Ring Connections

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