lmc6464amj-qml National Semiconductor Corporation, lmc6464amj-qml Datasheet - Page 14

• Manufacturer Part Number: lmc6464amj-qml
• Description: Lmc6464qml Quad Micropower, Rail-to-rail Input And Output Cmos Operational Amplifier
• Manufacturer: National Semiconductor Corporation

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7.0 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 LMC6464, typically 150 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 acceptably low, be-
cause under conditions of high humidity or dust or contami-
nation, the surface leakage will be appreciable.
To minimize the effect of any surface leakage, lay out a ring
of foil completely surrounding the LMC6464's inputs and the
terminals of capacitors, diodes, conductors, resistors, relay
terminals, etc. connected to the op-amp's inputs, as in
11. 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 be-
tween two points at the same potential. For example, a PC
board trace-to-pad resistance of 10
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 30 times degradation from the LMC6464's actual
performance. However, if a guard ring is held within 5 mV of
the inputs, then even a resistance of 10
0.05 pA of leakage current. See
tions of guard rings for standard op-amp configurations.
FIGURE 11. Example of Guard Ring in P.C. Board Layout
Figure 12
12
Ω, which is normally
11
Ω would cause only
for typical connec-
Figure
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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
(Input pins are lifted out of PC board and soldered directly to components.
All other pins connected to PC board.)
FIGURE 12. Typical Connections of Guard Rings
Non-Inverting Amplifier
FIGURE 13. Air Wiring
Inverting Amplifier
Follower
Figure
13.
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