The AD795 is designed for mounting on PC boards. Maintaining picoampere resolution in those environments requires a lot of care. Both the board and the amplifier’s package have finite resistance. Voltage differences between the input pins and other pins as well as PC board metal traces will cause parasitic currents (Figure 30) larger than the AD795’s input current unless special precautions are taken. Two methods of minimizing parasitic leakages are guarding of the input lines and maintaining adequate insulation resistance. Figures 31 and 32 show the recommended guarding schemes for follower and inverted topologies. Pin 1 is not connected, and can be safely connected to the guard. The high impedance input trace should be guarded on both edges for its entire length.