LMC6484 Datasheet, PDF (36/45 Pages) National Semiconductor (TI) – CMOS Quad Rail-to-Rail Input and Output Operational Amplifier

LMC6484
SNOS675C – AUGUST 2000 – REVISED SEPTEMBER 2015
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9 Power Supply Recommendations
The LMC6482 can be operated over a supply range of 3 V to 15 V. To achieve noise immunity as appropriate to
the application, it is important to use good printed circuit board layout practices for power supply rails and planes,
as well as using bypass capacitors connected between the power supply pins and ground.
10 Layout
10.1 Layout Guidelines
10.1.1 Printed-Circuit-Board Layout for High-Impedance Work
It is generally recognized that any circuit which must operate with less than 1000 pA of leakage current requires
special layout of the PC board. when one wishes to take advantage of the ultra-low input current of the
LMC6484, typically less than 20 fA, it is essential to have an excellent layout. Fortunately, 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 inputs
and the terminals of capacitors, diodes, conductors, resistors, relay terminals, etc., connected to the operational
amplifier inputs, as in Figure 78. 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 1012, which is normally considered a very large resistance, could leak 5 pA if
the trace were a 5-V bus adjacent to the pad of the input. This would cause a 250 times degradation from the
actual performance of the LMC6484. However, if a guard ring is held within 5 mV of the inputs, then even a
resistance of 1011 would cause only 0.05 pA of leakage current. See Figure 76 for typical connections of guard
rings for standard operational amplifier configurations.
Figure 76. 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: Do not insert the input
pin of the amplifier 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 77.
Note: (Input pins are lifted out of PC board and soldered directly to components. All other pins connected to PC board.)
Figure 77. Air Wiring
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