LMC6492 Datasheet, PDF (16/28 Pages) National Semiconductor (TI) – CMOS Rail-to-Rail Input and Output Operational Amplifier

LMC6492, LMC6494
SNOS724D – AUGUST 2000 – REVISED MARCH 2013
www.ti.com
Direct capacitive loading will reduce the phase margin of many op-amps. A pole in the feedback loop is created
by the combination of the op-amp's output impedance and the capacitive load. This pole induces phase lag at the
unity-gain crossover frequency of the amplifier resulting in either an oscillatory or underdamped pulse response.
With a few external components, op amps can easily indirectly drive capacitive loads, as shown in Figure 59.
Figure 59. LMC6492/4 Noninverting Amplifier, Compensated to Handle Capacitive Loads
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
LMC6492/4, typically 150 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 LMC6492/4's
inputs and the terminals of components connected to the op-amp's inputs, as in Figure 60. 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 33 times degradation from the LMC6492/4's actual performance. If a guard ring is used and
held within 5 mV of the inputs, then the same resistance of 10¹¹Î© will only cause 0.05 pA of leakage current. See
Figure 61 for typical connections of guard rings for standard op-amp configurations.
Figure 60. Examples of Guard Ring in PC Board Layout
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