How to measure pico amperes?!

Measuring pico amperes?!

Lower the current more the resistance required used in detecting-if this statement is right then how?If my first statement is true how are they able to realize very large resistance??:?:
please clarify my doubt

Re: Measuring pico amperes?!

Hi
Very large input resistance can be achieved by using FET and MOS. but in this case you ll alter the actual current in the measured circuit that is why we use moderate input resistnce amplifires to boost the voltage drop across that resistance and as you know that voltage is proportional to the measured current and the output voltage is proportional to the input voltage ie proportional to the input current but such testers are expensive .
Regards

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).

Re: Measuring pico amperes?!

Low currents are measured with current integrators. Δt is measured for set ΔV. Amplifiers with very high input impedance or specialized IC's are used for this. High value resistors will have high noise.