Multichannel, Low Nonlinearity Time-to-Digital Converters Based on 20 and 28 nm FPGAs

This paper presents low nonlinearity, compact, and multichannel time-to-digital converters (TDC) in Xilinx 28 nm Virtex 7 and 20 nm UltraScale field-programmable gate arrays (FPGAs). The proposed TDCs integrate several innovative methods that we have developed: 1) the sub-tapped delay line averaging topology; 2) tap timing tests; 3) a direct compensation architecture; and 4) a mixed calibration method. The code density tests show that the proposed TDCs have much better linearity performances than previously reported ones. Our approach is cost-effective in terms of the consumption of logic resources. To demonstrate this, we implemented 96 channel TDCs in both FPGAs, using less than 25% of the logic resources. The achieved least significant bit (LSB) is 10.5 ps for Virtex 7 and 5.0 ps for UltraScale FPGAs. After the compensation and calibration, the differential nonlinearity (DNL) is within [-0.05, 0.08] LSB with sigma DNL = 0.01 LSB, and the integral nonlinearity (INL) is within [-0.09, 0.11] LSB with sigma INL = 0.04 LSB for the Virtex 7 FPGA. The DNL is within [-0.12, 0.11] LSB with sigma DNL = 0.03 LSB, and the INL is within [-0.15, 0.48] LSB with sigma INL = 0.20 LSB for the UltraScale FPGA.