All-digital TAD-OFDM detection for sensor interface using TAD-digital synchronous detection

An all-digital TAD-OFDM detection method for sensor interface with digital synchronous detection based on TAD (Time A/D 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 V_in 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_s, this ADC (TAD) naturally performs as a V_in voltage-integration circuit for T_s. Hence, high frequency noises can be removed simultaneously with A/D conversion. First, as an example of a voltage-integration effect as a low-pass filter, magnet pick-up sensing was verified using a 22-bit TAD test chip fabricated using a 0.65-μm CMOS with 106 μV/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, TA-DOFDM 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.