AD795,pdf datasheet (Precision

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.