Piezoelectric nano-positioning stage approaches to the probing sphere through different step by PI controller. CCD is used to monitor the space between the stage and the probing sphere when micrometer screw mechanism is used for coarse adjustment. Capacitance change is detected by signal processing circuit which has interfacing with PC, the block diagram of the signal processing circuit is shown in Fig. 8, AD7747 chip is selected as a capacitanceto- digital converter, the input signal is very weak, so the circuit noise is an important factor to disturb the output, AD7747 provides capacitive input active AC shielding to eliminate the parasitic capacitance and shield cable is used as signal transmission line between capacitive sensor and AD7747. Furthermore,