LMC6482QML Datasheet, PDF (18/28 Pages) Texas Instruments – CMOS Dual Rail-To-Rail Input and Output Operational Amplifier

6.0 PRINTED-CIRCUIT-BOARD LAYOUT FOR HIGH-
IMPEDANCE WORK
It is generally recognized that any circuit which must operate
with less than 1000pA of leakage current requires special
layout of the PC board. When one wishes to take advantage
of the ultra-low input current of the LMC6482, typically less
than 20fA, 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 through it may sometimes appear ac-
ceptably low, because under conditions of high humidity or
dust or contamination, the surface leakage will be apprecia-
ble.
To minimize the effect of any surface leakage, lay out a ring
of foil completely surrounding the LM6482's inputs and the
terminals of capacitors, diodes, conductors, resistors, relay
terminals, etc. connected to the op-amp's inputs, as in Figure
9. 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 be-
tween two points at the same potential. For example, a PC
board trace-to-pad resistance of 10¹²Î©, which is normally
considered a very large resistance, could leak 5pA if the trace
were a 5V bus adjacent to the pad of the input. This would
cause a 250 times degradation from the LMC6482's actual
performance. However, if a guard ring is held within 5 mV of
the inputs, then even a resistance of 10¹¹Î© would cause only
0.05pA of leakage current. See Figure 10 for typical connec-
tions of guard rings for standard op-amp configurations.
Inverting Amplifier
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Non-Inverting Amplifier
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Follower
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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 construction, but
the advantages are sometimes well worth the effort of using
point-to-point up-in-the-air wiring. See Figure 11.
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FIGURE 9. Example of Guard Ring in P.C. Board Layout
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(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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