In this paper, an accurate front-end CMOS interface circuit for sensing very small capacitance changes in capacitive sensors is presented. The proposed structure scales capacitance variation to the sensible impedance changing. The scaling factor of the circuit can be easily tuned by adjusting bias points of the transistors. In order to cancel or decrease the parasitic components, the RC feedback and input transistor cascading techniques are employed in the design. To simulate the circuit, HSPICE simulator is utilized to verify the validity of the theoretical formulations in 0.18 mu m technology. According to schematic and post-layout simulation results, input impedance changes linearly versus capacitance variations up to 0.7 GHz, while the sensor capacitance changing is varied between 0 and 200 fF. According to the simulation results, total dc power consumption is obtained as low as 1 mW with 0.9 V power supply.