LMC662
SNOSC51C –APRIL 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 LMC662's inputs
and the terminals of capacitors, diodes, conductors, resistors, relay terminals, etc. connected to the op-amp's
inputs. See Figure 19. 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
LMC662'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 20, Figure 21, and Figure 22 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 23.
Figure 19. Example, using the LMC660,
of Guard Ring in P.C. Board Layout
Figure 20. Guard Ring Connections: Inverting Amplifier
Figure 21. Guard Ring Connections: Non-Inverting Amplifier
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