Spartan-6 FPGA based 8-channel time-to-digital converters for TOF-PET systems

According to the technical requirement of the time coincident measurement for TOF-PET systems, an eight-channel wave-union time-to-digital convertor (TDC) based on a Spartan-6 Xilinx FPGA has been implemented and tested. The root-mean-square (RMS) error of the two channels on the same FPGA measuring a fixed time interval is less than 14 ps, while the RMS error of the two channels inter-FPGA is less than 26.5 ps. The TDC measurement dead time is only 8 ns, which is conducive to the trigger mechanism combining a low threshold for timing and a high threshold for event confirmation. Because the time coincidence measurements in PET scanners always occur between the opposing TDC circuits, the variation of the RMS error and measured mean value by the two inter-FPGA TDC channels in different temperature environments is also presented.