DC current sources and meters at femto-amperes level are essential for low-level current applications, such as leakage current measurement for electrical/electronic component, current measurement for mass spectrometer, measurements of test probe current in electrochemistry as well as calibration of and precision medical equipment. Conventional method using test current generated by high-valued resistor (i.e. 1×109 Ω or above) is not suitable for this application in terms of its levels of precision and accuracy.