the input terminals from the other pins. The solder mask was also
removed from this area to reduce surface charge accumulation. The
guard generator was a buffer amplifier which drove the input
guard traces to the same potential as the inverting input node.
In order to shield the circuit from electromagnetic interference,
the entire circuit was enclosed within a metal shield box and the
input signal was connected to the FC using a triax cable. The inside
shield of the triax cable was connected to the guard terminal of the
picoammeter circuit so that the signal conduct and the inside
shield were at the same potential. This approach eliminates the
considerable charge injected by traditional coaxial-cables.