Any circuit that must operate with < 1000pA of leakage current requires special layout of the PCB. To take advantage of the ultra-low bias current of the LMC6035-Q1 (typically < 0.04pA), an excellent layout is essential. Fortunately, the techniques for obtaining low leakages are quite simple. First, do not ignore the surface leakage of the PCB, even though at times the surface leakage can 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 LMC6035-Q1 inputs and the terminals of capacitors, diodes, conductors, resistors, relay terminals, and so on, connected to the op-amp inputs. See also Figure 7-14. To have a significant effect, place guard rings on both the top and bottom of the PCB. This PCB 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 an input. This configuration can cause a 100 times degradation from the actual performance of the amplifier. 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, or perhaps a minor (2:1) degradation of the amplifier performance. See Figure 7-10 through Figure 7-12 for typical connections of guard rings for standard op-amp configurations. If both inputs are active and at high impedance, the guard can be tied to ground and still provide some protection; see also Figure 7-13.

Figure 7-10 Guard Ring Connections: Inverting Amplifier

Figure 7-12 Guard Ring Connections: Follower

Figure 7-11 Guard Ring Connections: Noninverting Amplifier

Figure 7-13 Howland Current Pump

A more comprehensive discussion on high impedance circuit design and considerations, see also Measurement and Calibration Techniques for Ultra-low Current Measurement Systems application note.