LMC660EP Datasheet, PDF (10/15 Pages) National Semiconductor (TI) – CMOS Quad Operational Amplifier

Application Hints (Continued)
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.
(a) Inverting Amplifier
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FIGURE 5. Example, using the LMC660AIMEP,
of Guard Ring in P.C. Board Layout
(b) Non-Inverting Amplifier
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(c) Follower
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(d) Howland Current Pump
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FIGURE 6. Guard Ring Connections
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.
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