LMC6001-MIL Datasheet, PDF (14/18 Pages) Texas Instruments – Ultra, Ultra-Low Input Current Amplifier

LMC6001-MIL
SNOSD60 – JUNE 2017
www.ti.com
8 Power Supply Recommendations
See the Recommended Operating Conditions for the minimum and maximum values for the supply input voltage
and operating junction temperature.
9 Layout
9.1 Layout Guidelines
9.1.1 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 PCB. When one wishes to take advantage of the ultra-low bias current of the LMC6001,
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 PCB, 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 inputs of the
LMC6001 and the terminals of capacitors, diodes, conductors, resistors, relay terminals, and so forth, connected
to the inputs of the op amp, as in Figure 30. To have a significant effect, guard rings must be placed on both the
top and bottom of the PCB. This PC foil must then be connected to a voltage which is at the same voltage as the
amplifier inputs, because no leakage current can flow between two points at the same potential. For example, a
PCB trace-to-pad resistance of 10 TΩ, which is normally considered a very large resistance, could leak 5 pA if
the trace were a 5-V bus adjacent to the pad of the input.
This would cause a 500 times degradation from the LMC6001's actual performance. If a guard ring is used and
held within 1 mV of the inputs, then the same resistance of 10 TΩ will only cause 10 fA of leakage current. Even
this small amount of leakage will degrade the extremely low input current performance of the LMC6001. See
Figure 28 for typical connections of guard rings for standard op amp configurations.
Figure 26. Inverting Amplifier
Figure 27. Noninverting Amplifier
Figure 28. Typical Connections of Guard Rings
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