The AD795 is designed for mounting on PC boards. Maintainingpicoampere resolution in those environments requires a lot ofcare. Both the board and the amplifier’s package have finiteresistance. Voltage differences between the input pins and otherpins as well as PC board metal traces will cause parasitic currents(Figure 30) larger than the AD795’s input current unless specialprecautions are taken. Two methods of minimizing parasiticleakages are guarding of the input lines and maintaining adequateinsulation resistance.Figures 31 and 32 show the recommended guarding schemesfor follower and inverted topologies. Pin 1 is not connected, andcan be safely connected to the guard. The high impedance inputtrace should be guarded on both edges for its entire length.