Abstract：An improved two terminal-pair coaxial AC bridge with auto source balance was constructed at NIM for high precision capacitance measurement. The errors introduced by auto source balance in the bridge are at negligible level. The bridge can disseminate the calculable capacitance to 1 pF, and transmit capacitance from 1 pF to 100 pF. An improved bootstrap method to calibrate the IVD ratios was adopted. A minimum standard uncertainty 5×10-9 was achieved at 1 592 Hz. The two terminal-pair capacitance bridge was applied to participate in the international key comparison of 10 pF and 100 pF capacitance standards, the comparison results showed that NIM's comparison results for 10 pF and 100 pF achieved a very good agreement with the KCRVs (key comparison recommended value), which obtained the international mutual recognition.
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