A multi-measurements RO-TDC implemented in a Xilinx field programmable gate array

In this paper, an area efficient time to digital converter (TDC) performing measurements between multiple hit signals is proposed. Our TDC is based on a delay line configured as a ring oscillator and a round tracker to count the number of iterations through the oscillator. Lookup tables configured as distributed RAMs and shift registers are used to sample the oscillator and the round tracker states whenever a transition on a signal occurs. A theoretical study is elaborated to estimate FPGA resources required to implement the proposed TDC in comparison with a multichannel basic RO-TDC. It is shown that the gain in the number of Flip-Flops and Lookup tables can reach factors of 85 and 2.1 respectively in an architecture made of a six-stage oscillator, a 32-state round tracker and 20 input hit signals. Temporal characteristics extracted from our TDC implemented in a Xilinx ZYNQ family FPGA are reported.