Noise coupled by external sources could be reduced with guarding
and shielding techniques. Guarding is an important technique
in sub-picoamp current designs. A guard is a low impedance point
in the circuit that is at nearly the same potential as the high impedance
lead being guarded. On the PCB layout in Fig. 4, the guard creates
an equal potential zone around the input pins, preventing
external leakage currents from coupling into the input circuit