LMC6032 Datasheet, PDF (8/13 Pages) National Semiconductor (TI) – CMOS Dual Operational Amplifier

Application Hints (Continued)
than 0.04 pA, it is essential to have an excellent layout. For-
tunately, the techniques for 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 LMC6032’s inputs and the
terminals of capacitors, diodes, conductors, resistors, relay
terminals, etc. connected to the op-amp’s inputs. See Figure
5. 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 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 an input. This
would cause a 100 times degradation from the LMC6032’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, or perhaps a minor
(2:1) degradation of the amplifier’s performance. See Figure
6a, Figure 6b, Figure 6c for typical connections of guard
rings for standard op-amp configurations. If both inputs are
active and at high impedance, the guard can be tied to
ground and still provide some protection; see Figure 6d.
(a) Inverting Amplifier
DS011135-7
(b) Non-Inverting Amplifier
DS011135-8
(c) Follower
DS011135-9
DS011135-6
FIGURE 5. Example of Guard Ring in
P.C. Board Layout
DS011135-10
(d) Howland Current Pump
FIGURE 6. Guard Ring Connections
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 in-
sulator. 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 7.
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