A Two-Step Time-to-Digital Converter With 5.6-ps Resolution and 1-4255-μs Measurement Range.

This paper presents a two-step coarse-fine time-to-digital converter (TDC) for ultrasonic flowmeter applications. To achieve a high resolution with a low frequency reference clock, a 7.7-MHz reference frequency is multiplied by 32 times using a multiplying delay-locked loop (MDLL) to generate a high-speed internal clock for coarse quantization. The resulting residual time error after coarse quantization is pulse-stretched and finely quantized. A cost-effective calibration scheme is proposed to ensure the PVT robustness of the pulse stretching circuit. The proposed TDC is realized in a 0.11μm/1.5V CMOS technology. It achieves a resolution of 5.6ps with a wide input range from 1 to 4255μs. The core TDC circuit occupies 0.065 mm and consumes 1.8mW. The measured differential nonlinearity and integral nonlinearity is ±3.1ps and ±59.7ps respectively.