LMC6081 Datasheet, PDF (11/21 Pages) National Semiconductor (TI) – Precision CMOS Single Operational Amplifier

LMC6081
www.ti.com
SNOS649C – AUGUST 2000 – REVISED MARCH 2013
Capacitive load driving capability is enhanced by using a pull up resistor to V⁺ (Figure 26). Typically a pull up
resistor conducting 500 μA or more will significantly improve capacitive load responses. The value of the pull up
resistor must be determined based on the current sinking capability of the amplifier with respect to the desired
output swing. Open loop gain of the amplifier can also be affected by the pull up resistor (see electrical
characteristics).
Figure 26. Compensating for Large
Capacitive Loads with a Pull Up Resistor
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
LMC6081, typically less than 10 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, 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 LMC6081's inputs
and the terminals of capacitors, diodes, conductors, resistors, relay terminals, etc. connected to the op-amp's
inputs, as in Figure 27. 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 normally 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
LMC6081'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 28 for typical connections of guard
rings for standard op-amp configurations.
Figure 27. Example of Guard Ring in P.C. Board Layout
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