An all-digital TAD-OFDM detection method for sensor interface with digital synchronous detection based on TAD (Time AID converter)-type ADC is introduced. A TAD basic structure is a completely digital circuit including a ring delay-line (RDL) with delay units (DUs) driven by an input voltage Vio and an RDL frequency counter, latch and encoder. Since the operating principle is to count the number of DUs through which the delay pulse passes within a sampling period T., this ADC (TAD) naturally performs as a Vio voltage integration circuit for T •. Hence, high frequency noises can be removed simultaneously with AID conversion. First, as an example of a voltage-integration effect as a low-pass tiIter, magnet pick-Up sensing was verified using a 22-bit TAD test chip fabricated using a 0.65-Jlm CMOS with 106 JlV/LSB (100 kS/s). Next, the idea of digital synchronous detection with TAD (called TAD-DSD with the voltage-integration effect) is proposed. Finally, by using the TAD-DSD principle, TAD OFDM detection is presented and experimentally confirmed, resulting in signal-detection of each wave-amplitude from a four-carrier-composite wave, which consists of frequencies, 1.6-, 3.2-, 6.4-and 12.8-MHz, respectively.