This paper describes a CMOS transponder IC for RFID applications. A full-wave rectifier implemented using NMOS transistors supplies the transponder with a dc supply voltage. A 64-bit ROM has been designed for a data memory. Front-end impedance modulation and Manchester coding are used for transmitting the data from the transponder memory to the reader. A new damping circuit has been proposed and employed for impedance modulation. The designed circuit has been fabricated using a 0.65 µm 2-poly, 2-metal CMOS process. Measurement results show that it has a constant damping rate of around 20-25% and a data transmission rate of 3.9 kbps at a 125 kHz RF carrier. Die area is 0.9 mm0.4 mm. The measured reading distance is up to 7 cm.