As a general rule, any circuit that must operate with less than 1000pA of leakage current requires special layout of the printed circuit board (PCB). When one wishes to take advantage of the ultra-low input current of the LMC646x, typically 150fA, an excellent layout is essential. Fortunately, the techniques of obtaining low leakages are quite simple. First, do not ignore the surface leakage of the PCB, even though the leakage current can sometimes appear acceptably low, because under conditions of high humidity or dust or contamination, the surface leakage can be appreciable.

To minimize the effect of any surface leakage, lay out a ring of foil completely surrounding the LMC646x inputs and the terminals of capacitors, diodes, conductors, resistors, relay terminals, and so forth connected to the inputs of the op amp, as in Figure 7-20. To have a significant effect, place guard rings on both the top and bottom of the PCB. This PC foil must then be connected to a voltage that 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 10¹², which is normally considered a very large resistance, can leak 5pA if the trace were a 5V bus adjacent to the pad of the input. This leakage can cause a 250 times degradation from the actual performance of the LMC646x. However, if a guard ring is held within 5mV of the inputs, then even a resistance of 10¹¹Ω causes only 0.05pA of leakage current. See Figure 7-21 through Figure 7-23 for typical connections of guard rings for standard op-amp configurations

Figure 7-20 Example of Guard Ring in P.C. Board Layout

Figure 7-21 Typical Connections of Guard Rings – Inverting Amplifier

Figure 7-22 Typical Connections of Guard Rings – Non-Inverting Amplifier

Figure 7-23 Typical Connections of Guard Rings – Follower

Be aware that when laying out a PCB for the sake of just a few circuits is not practical, another technique is even better than a guard ring on a PCB: Do not insert the input pin of the amplifier into the PCB at all, but bend the pin up in the air, and use only air as an insulator. Air is an excellent insulator. In this case you forgo some of the advantages of PCB construction, but the advantages are sometimes well worth the effort of using point-to-point up-in-the-air wiring. See Figure 7-24.

(Input pins are lifted out of PCB and soldered directly to components. All other pins connected to PCB.)

Figure 7-24 Air Wiring