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Description
High resolution time-to-digital converter (TDC) is widely used in high-energy physics experiments, TOF-PET, and other fields. Targeting next-generation photon detectors (MCP-PMT, SiPM), this work presents a TDC prototype design achieving state-of-the-art time resolution in a 55-nm CMOS process.
The design integrates two timing cores for independent time-of-arrival (TOA) and time over threshold (TOT) measurements. Each core has four primary components: two ring oscillators (VRO) based on a vernier structure, a controller, a quantization block and counters. The VRO includes a slow ring (X-chain) and a fast ring (Y-chain), both employing 15 delay units. Each delay unit adopts a voltage-controlled starved-current-mirror NAND gate. The time resolution is determined by the difference between the unit delays of the X chain and Y chain.
The TDC employs event-driven logic to reduce power consumption. Upon event detection, the controller segments the pulse into START and STOP signals to enable both VRO rings. The SR-latch array in the quantization block synchronously captures phases states across all NAND outputs, comparing each rising and falling edges. Measurement terminates when X-chain and Y-chain phase align, latching the corresponding 30-bit thermometer code. The VRO enters low-power standby mode after quantization until reactivated by subsequent events. A 10-bit coarse counter and a 10-bit fine counter provide a dynamic range of 2.4 μs. Finally, the two cores generate 100-bit raw data, which is serialized and transmitted through a serializer working at 640 MHz.
The post-layout simulations demonstrate a fitting resolution of 1.77 ps under typical condition. The differential nonlinearity (DNL) and integral nonlinearity (INL) is within ±1 LSB and ±2 LSB, respectively. The TDC consumes 8 mW during active operation and 0.72 mW in standby mode after event completion. Preliminary tests confirm correct operation of the VRO rings with achievable time resolution ranging from 5 ps to 36 ps. Additional testing is currently underway, with results to be reported subsequently.
