Re: Measuring pico amperes?!
Low currents can be measured with an electrometer amplifier and a high megohm feedback resistor. Typical feedback resistors are in the range 100 to 1000 MΩ. This is often used for measuring the small currents produced in ionization chambers, etc.
The electrometer amplifier summing junction can't be connected directly to the circuit board because there is too much leakage current across the board surface, even with guarding. To get around this problem, you can use something like an elevated, teflon-insulated binding post that is secured to the board but makes no electrical contact with it. The post serves as a mechanically stable point to connect together the amplifier summing input, high megohm resistor, and the device that is producing the current (e.g., ion chamber).