Due to the high impedance of a modern amplifier, the circuit has an ultra-high impedance node, and the circuit is sensitive to the movement of the person or an object near the instrument that creates unwanted electrical coupling.

Figure 3-6 Coulombmeter With the High Impedance Node in Green

In addition, AC fields, such as the mains line, also influences high-impedance node. When the high-impedance node couples with mains lines with a parasitic capacitance of 1fF, the resulting error current can be 42pA.

Figure 3-7 Aluminum Box for Shielding High-Impedance Node From EMI

An aluminum box shields the high-impedance node from electronic field coupling. Depending on the type of radiation, different material is selected. For inductive coupling, higher permeability material is required.

Figure 3-8 Vout With a Shield

Figure 3-9 Vout Without Shield