Improve small capacitance measurement sensitivity by opto-electronic isolation method

The cross capacitor of NIM established in 1970 s is the primary standard of Chinese capacitance traceability system. It has a relative uncertainty of 1 in 10⁷. The value of the capacitor is about 0.5 pF. This small value leads to a low SNR, and the sensitivity of the measurement is about several parts in 10⁸ . An opto-electronic isolating method is designed in this paper, cooperated with a lock-in amplifier, which could improve the capacitance measurement sensitivity to 1 in 10⁸.