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This paper presents the design of a coarse-fine time-to-digital converter (TDC) that amplifies a time residue to improve time resolution, similar to a coarse-fine analog-to-digital converter (ADC). A new digital circuit has been developed to amplify the time residue with a higher gain (>16) and larger range (>80 ps) than existing solutions do. However, adapting the conventional coarse-fine architecture from ADCs is not an appropriate solution for TDCs: input time cannot be stored, and the gain of a time amplifier (TA) cannot be controlled precisely. This paper proposes a new coarse-fine TDC architecture by using an array of time amplifiers and two identical fine TDCs that compensate for the variation of the TA gain during the conversion process. The measured DNL and INL are plusmn0.8 LSB and plusmn3 LSB, respectively, with a value of 1.25 ps per 1 LSB, while the standard deviation of output code for constant inputs remains below 1 LSB across the TDC range. Although the nonlinearity is larger than 1 LSB, using an INL lookup table or better matched delays in the coarse TDC delay chain will improve the linearity further.
Date of Publication: April 2008