Generally, any circuit that must operate with less than 1000pA of leakage current requires special layout of the printed circuit board (PCB). To take advantage of the ultra-low bias current of the LMC606x, typically less than 10fA, having an excellent layout is essential. Fortunately, the techniques used to obtain low leakages are quite simple. First, do not ignore the surface leakage of the PCB, even though the leakage can sometimes appear acceptably low. Under conditions of high humidity, 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 LMC606x inputs and the terminals of capacitors, diodes, conductors, resistors, relay terminals, and so on, connected to the op amp inputs, as in Figure 6-11. To have a significant effect, place guard rings on both the top and bottom of the PCB. Then connect this foil 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 10¹²Ω, which is normally considered a very large resistance, can leak 5pA if the trace is a 5V bus adjacent to the pad of the input. This leak can cause a 100 times degradation from the LMC606x actual performance. 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 6-7 to Figure 6-9 for typical connections of guard rings for standard op amp configurations.

Figure 6-7 Typical Connections of Guard Rings: Inverting Amplifier

Figure 6-9 Typical Connections of Guard Rings: Follower

Figure 6-8 Typical Connections of Guard Rings: Noninverting Amplifier

When laying out a PCB for the sake of just a few circuits is not practical, the following technique is even better than a guard ring. Do not insert the input pin of the amplifier into the PCB at all. Instead, 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. Figure 6-10 shows an example of air wiring.

Note: Input pins are lifted out of the PCB and soldered directly to components. All other pins are connected to the PCB.

Figure 6-10 Air Wiring