Application Information (Continued)
6.0 PRINTED-CIRCUIT-BOARD LAYOUT FOR
HIGH-IMPEDANCE WORK
It is generally recognized that any circuit which must oper-
rate with less than 1000pA of leakage current requires spe-
cial layout of the PC board. When one wishes to take ad-
vantage of the ultra-low input current of the LMC6482,
typically less than 20fA, 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 through it may sometimes
appear acceptably low, because under conditions of high
humidity or dust or contamination, the surface leakage will
be appreciable.
01171321
Inverting Amplifier
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 Fig-
ure 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 between two points at the same potential. For example,
a PC board trace-to-pad resistance of 1012Ω, which is nor-
mally 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 1011Ω would
cause only 0.05pA of leakage current. See Figure 10 for
typical connections of guard rings for standard op-amp con-
figurations.
01171322
Non-Inverting Amplifier
01171323
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.
01171320
FIGURE 9. Example of Guard Ring in P.C. Board
Layout
01171324
(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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