Any circuit that operates 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 LMC649x, typically 150fA, an excellent layout is required. Fortunately, the techniques to obtain low leakages are quite simple. First, do not ignore the surface leakage of the PCB. Even though this leakage can sometimes appear acceptably low, under conditions of high humidity or dust or contamination, the surface leakage is appreciable.

To minimize the effect of any surface leakage, lay out a ring of foil completely surrounding the LMC649x inputs and the terminals of components connected to the op amp inputs, as in Figure 6-15. To have a significant effect, place guard rings on both the top and bottom of the PCB. This printed circuit 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 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 causes a 33 times degradation from the LMC649x actual performance. If a guard ring is used and held within 5mV of the inputs, then the same resistance of 10¹¹Ω only causes 0.05pA of leakage current. Figure 6-15 to Figure 6-17 show typical connections of guard rings for standard op-amp configurations.

Figure 6-15 Examples of Guard Ring in PCB Layout

Figure 6-16 Inverting Amplifier

Figure 6-18 Follower

Figure 6-17 Noninverting Amplifier

Be aware that when laying out a PCB for the sake of just a few circuits is inappropriate, the following technique is even better than a guard ring on a PCB. Do not insert the amplifier input pin into the board at all; instead, bend the input pin up in the air and use only air as an insulator because air is an excellent insulator. In this case, some of the advantages of PCB construction are lost, but the advantages of air are sometimes well worth the effort of using point-to-point up-in-the-air wiring. Figure 6-19 shows an example of air wiring.

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

Figure 6-19 Air Wiring