LMC6572 Datasheet, PDF (9/13 Pages) National Semiconductor (TI) – Low Voltage (2.7V and 3V) Operational Amplifier

Applications Hints (Continued)
nent of the output signal back to the amplifier’s inverting in-
put, thereby preserving phase margin in the overall feedback
loop.
4.0 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 bias current of the LMC6574/2, typically less
than 20 fA, 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 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 LMC6574/2’s inputs and
the terminals of capacitors, diodes, conductors, resistors, re-
lay terminals, etc. connected to the op-amp’s inputs, as in
Figure 3. 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 the input. This
would cause a 250 times degradation from the LMC6574/2’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. See Figure 4 for typi-
cal connections of guard rings for standard op-amp
configurations.
Inverting Amplifier
DS011934-9
Non-Inverting Amplifier
DS011934-10
DS011934-11
Follower
FIGURE 4. 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 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
5.
DS011934-8
FIGURE 3. Example of Guard Ring in P.C. Board
Layout
DS011934-12
(Input pins are lifted out of PC board and soldered directly to components.
All other pins connected to PC board).
FIGURE 5. Air Wiring
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