LMC6484AIMX Datasheet, PDF (14/24 Pages) Texas Instruments – LMC6484 CMOS Quad Rail-to-Rail Input and Output Operational Amplifier

Application Information (Continued)
of the ultra-low input current of the LMC6484, typically less
than 20 fA, it is essential to have an excellent layout. Fortu-
nately, the techniques of obtaining low leakages are quite
simple. First, the user must not ignore the surface leakage of
the PC board, even though it may sometimes appear accept-
ably low, because under conditions of high humidity or dust
or contamination, the surface leakage will be appreciable.
To minimize the effect of any surface leakage, lay out a ring
of foil completely surrounding the LMC6484’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 in 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 10¹²â„¦, 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 LMC6484’s
actual performance. However, if a guard ring is held within 5
mV of the inputs, then even a resistance of 10¹¹â„¦ would
cause only 0.05 pA of leakage current. See Figure 10 for
typical connections of guard rings for standard op-amp
configurations.
Inverting Amplifier
DS011714-21
Non-Inverting Amplifier
DS011714-22
DS011714-20
FIGURE 9. Example of Guard Ring in P.C. Board
Layout
DS011714-23
Follower
FIGURE 10. Typical Connections of Guard Rings
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 11.
DS011714-24
(Input pins are lifted out of PC board and soldered directly to components.
All other pins connected to PC board.)
FIGURE 11. Air Wiring
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