A TDC-based 4×128 CMOS SPAD array for time-gated Raman spectroscopy

A 4×128 CMOS SPAD array has been designed and tested for a time-gated Raman spectroscopy. The SPAD array includes a 3-bit TDC for each of the SPAD elements (512). The TDC is designed to have 78 ps resolution in the first four bins for the accurate time position measurement of the Raman photons. The resolution is degraded for the last four bins used for fluorescence and background correction to achieve larger dynamic range. The timing skew of the bins of the TDC along spectral axis was measured to be ± 70 ps with 7 ps variation (sigma) in the bin widths.