LMC6034
SNOS608C –MAY 1998–REVISED MARCH 2013
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To minimize the effect of any surface leakage, lay out a ring of foil completely surrounding the LMC6034's inputs
and the terminals of capacitors, diodes, conductors, resistors, relay terminals, etc. connected to the op-amp's
inputs. See Figure 17 . 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 1012Ω, which is normally 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
LMC6034's actual performance. However, if a guard ring is held within 5 mV of the inputs, then even a
resistance of 1011Ω would cause only 0.05 pA of leakage current, or perhaps a minor (2:1) degradation of the
amplifier's performance. See Figure 18, Figure 19 and , Figure 20 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 21.
Figure 17. Example of Guard Ring in P.C. Board Layout
Figure 18. Guard Ring Connections Inverting Amplifier
Figure 19. Guard Ring Connections Non-Inverting Amplifier
Figure 20. Guard Ring Connections Follower
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