This circuit uses a CA3420 BiMOS op amp to form a picoammeter with 4 ranges. The exceptionally low input current (typically 0.2pA) makes the CA3420 highly suited for use in a picoammeter circuit. Input transient protection is provided by the 1 megohm resistor in series with the input. The 10 megohm resistor connected to pin 2 decouples the potentially high input capacitance often associated with lower current circuits and reduces the tendency for the circuit to oscillate under these conditions. The 10k potentiometer is used for null offset.
Source: Electronics Lab