Efficient time-to-digital converters in 20 nm FPGAs with wave union methods

The wave union (WU) method is a well-known method in time-to-digital converters (TDCs) and can improve TDC performances without consuming extra logic resources. However, a famous earlier study concluded that the WU method is not suitable for UltraScale field-programmable gate array (FPGA) devices, due to more severe bubble errors. This paper proves otherwise and presents new strategies to pursue high-resolution TDCs in Xilinx UltraScale 20 nm FPGAs. Combining our new sub-tapped delay line (sub-TDL) architecture (effective in removing bubbles and zero-width bins) and the WU method, we found that the wave union method is still powerful in UltraScale devices. We also compared the proposed TDC with the TDC combining the dual sampling (DS) structure and the sub-TDL technique. A binning method is introduced to improve the linearity. Moreover, we derived a formula of the total measurement uncertainties for a single-stage TDL-TDC to obtain its root-mean-square (RMS) resolution. Compared with previously published FPGA-TDCs, we presented (for the first time) much more detailed precision analysis for single-TDL TDCs.