LMC6064EP Datasheet, PDF (12/18 Pages) Texas Instruments – Precision CMOS Quad Micropower Operational Amplifier

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 LMC6064EP, typically less
than 10 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 LMC6064EP's inputs and
the terminals of capacitors, diodes, conductors, resistors, re-
lay terminals etc. connected to the op-amp's inputs, as in
Figure 4. 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 10¹²Î©, 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 100 times degradation from the LMC6064EP's
actual performance. However, if a guard ring is held within
5 mV of the inputs, then even a resistance of 10¹¹Î© would
cause only 0.05 pA of leakage current. See Figure 5 for typical
connections of guard rings for standard op-amp configura-
tions.
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FIGURE 4. Example of Guard Ring in P.C. Board Layout
Latchup
CMOS devices tend to be susceptible to latchup due to their
internal parasitic SCR effects. The (I/O) input and output pins
look similar to the gate of the SCR. There is a minimum cur-
rent required to trigger the SCR gate lead. The LMC6064EP
Inverting Amplifier
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Non-Inverting Amplifier
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Follower
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FIGURE 5. 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 construction, but
the advantages are sometimes well worth the effort of using
point-to-point up-in-the-air wiring. See Figure 6.
and LMC6082 are designed to withstand 100 mA surge cur-
rent on the I/O pins. Some resistive method should be used
to isolate any capacitance from supplying excess current to
the I/O pins. In addition, like an SCR, there is a minimum
holding current for any latchup mode. Limiting current to the
supply pins will also inhibit latchup susceptibility.
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201144 Version 2 Revision 3 Print Date/Time: 2011/08/30 23:14:26
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