Radiation-Tolerant CMOS Timing Readout Circuits for Laser Detection and Ranging in Nuclear Reactors

A novel multi-stage noise-shaping (MASH) delta-sigma (∆Σ) Time-to-Digital Converter (TDC) structure is proposed for applications in continuous-time pulsed time-offlight (TOF) rangefinders for nuclear reactor remote sensing, which requires both high resolution and multi MGy gamma-dose radiation tolerance. The converter, implemented in 0.13 μm CMOS, achieves a time resolution of 5.6 ps and an ENOB of 11 bits. The TDC core consumes only 1.7 mW and occupies an area of 0.11 mm. Owing to the usage of circuit level radiation hardened-by-design techniques, such as passive RC oscillators and constant-gm biasing, the TDC exhibits enhanced radiation tolerance. After a total dose of 3.4 MGy at a high dose rate of 30 kGy/h, the TDC still achieves a time resolution of 10.5 ps. In order to secure the temperature stability of the TDC, a bandgap reference is employed to provide reference current and voltage for the TDC core. The total-ionizing-dose (TID) radiation tolerance of bandgap references in deep-submicron CMOS technology is generally limited by the radiation introduced leakage current in diodes. An analysis of this phenomenon is given in this paper, and a dynamic base leakage compensation (DBLC) technique is proposed to improve the radiation hardness of a bandgap reference built in a standard 0.13 μm CMOS technology. A temperature coefficient of 15 ppm/°C from −40 °C to 125 °C is measured before irradiation. The voltage variation from 0 °C to 100 °C is only ±1 mV for an output voltage of 600 mV. Gamma irradiation assessment approves that the bandgap reference is tolerant to a total ionizing dose of at least 4.5 MGy. The output reference voltage exhibits a variation of less than 1 % during the entire experiment, when the chip is irradiated by gamma ray at a dose rate of 27 kGy/h.