6.0 Printed-Circuit-Board Layout for High-Impedance

Work

It is generally recognized that any circuit which must oper-

rate with less than 1000 pA of leakage current requires spe-

cial layout of the PC board. When one wishes to take advan-

tage of the ultra-low input current of the LMC6482, typically

less than 20 fA, it is essential to have an excellent layout.

Fortunately, the techniques of obtaining low leakages are

quite simple. First, the user must not ignore the surface leak-

age of the PC board, even through it may sometimes appear

acceptably low, because under conditions of high humidity or

dust or contamination, the surface leakage will be appre-

ciable.

To minimize the effect of any surface leakage, lay out a ring

of foil completely surrounding the LM6482’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 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 LMC6482’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.

DS011713-20

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

Layout

DS011713-21

Inverting Amplifier

DS011713-22

Non-Inverting Amplifier

DS011713-23

Follower

FIGURE 10. Typical Connections of Guard Rings

DS011713-24

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

All other pins connected to PC board.)

FIGURE 11. Air Wiring

www.national.com

14