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

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 of using point-to-point up-in-the-air wiring. See Figure

7.

00876716

FIGURE 5. Example, using the LMC660,

of Guard Ring in P.C. Board Layout

00876717

(a) Inverting Amplifier

00876718

(b) Non-Inverting Amplifier

00876719

(c) Follower

00876720

(d) Howland Current Pump

FIGURE 6. Guard Ring Connections

www.national.com

9