A Two-Stage Interpolation Time-to-Digital Converter Implemented in 20 and 28 nm FGPAs

This article presents a two-stage interpolation time-to-digital converter (TDC), combining a Vernier gray code oscillator TDC (VGCO-TDC) and a tapped-delay line TDC (TDL-TDC). The proposed TDC uses the Nutt method to achieve a broad, high-resolution measurement range. It utilizes look-up tables based gray code oscillators to build a VGCO-TDC as the first-stage interpolation for fine-time measurements. Then the overtaking residual from the VGCO-TDC is measured by a TDL-TDC to achieve the second-stage interpolation. Due to the two-stage interpolation architecture, the carry-chain-based delay line only needs to cover the resolution of the VGCO-TDC. Hence, we can reduce the delay-line length and related hardware resource utilization. We implemented and evaluated a 16-channel TDC system in Xilinx 20-nm Kintex-UltraScale and 28-nm Virtex-7 field-programmable gate arrays. The Kintex-UltraScale version achieves an average resolution [least significant bit (LSB)] of 4.57 picoseconds (ps) with 4.36 LSB average peak-to-peak differential nonlinearity (DNL
_pk-pk
). The Virtex-7 version achieves an average resolution of 10.05 ps with 2.85 LSB average DNL
_pk-pk
.