Abstract: AC weak current measurement is one of the key technologies in AC impedance measurement. Effect of the AC weak current measurement to AC impedance measurement is analyzed, the method of using Phase Sensitive Detector (PSD) to measure the AC weak current  is put forwarded, using cross-correlation technique the amplifier noise influence to AC current measurement  is conquered, the expected function of AC impedance measurement system is done and  the desired accuracy is achieved.

Key words: AC measurement, Micro current, Impedance, Phase sensitive detector, Weak-signal detective