An FPGA-Based Time-to-Digital Converter With Online Dual-Chain Calibration

This article introduces a field-programmable gate array (FPGA)-based time-to-digital converter (TDC) with an online dual-chain calibration method to address time resolution and linearity issues arising from the non-uniformity of FPGA internal delay elements. The aim is to enhance the stability and accuracy of TDCs under varying environmental conditions. We employ an external high-precision clock to generate start-stop calibration signals for real-time monitoring of signal propagation time changes within FPGA components. Two identical delay chains in the TDC are managed by a finite state machine (FSM) to switch their roles, ensuring continuous data acquisition and online calibration. Experimental results show that the proposed TDC significantly outperforms traditional time interval counter (TIC) in terms of time linearity, maximum integral nonlinearity (INL), and differential nonlinearity (DNL). Within a temperature range of $20~^{\circ }$ C– $80~^{\circ }$ C, post-online temperature calibration maintains the root mean square (rms) of the TDC below 20 ps, substantially improving performance across different temperature conditions.