A Novel FPGA Implementation of a Time-to-Digital Converter Supporting Run-Time Estimation and Compensation

Time-to-digital converters (TDCs) are widely used in applications that require the measurement of the time interval between events. In previous designs using a feedback loop and an extended delay line, process-voltage-temperature (PVT) variation often decreases the accuracy of measurements. To overcome the loss of accuracy caused by PVT variation, this study proposes a novel design of a synthesizable TDC that employs run-time estimation and compensation of PVT variation. A delay line consisting of a series of buffers is used to detect the period of a ring oscillator designed to measure the time interval between two events. By comparing the detected period and the system clock, the variation of the oscillation period is compensated at run-time. The proposed TDC is successfully implemented by using a low-cost Xilinx Spartan-6 LX9 FPGA with a 50-MHz oscillator. Experimental results show that the proposed TDC is robust to PVT variation with a resolution of 19.1 ps. In comparison with previous design, the proposed TDC achieves about five times better tradeoff in the area, resolution, and frequency of the reference clock.