A Wide Range, 4.2 ps(rms) Precision CMOS TDC With Cyclic Interpolators Based on Switched-Frequency Ring Oscillators

A time-to-digital (TDC) converter based on cyclic interpolators has been designed. The TDC is designed to be used in a pulsed time-of-flight laser radar, where a long measurement range is required. The TDC's two interpolators provide a picosecond level resolution, which is combined with a main reference clock counter to give a measurement range of 327 μs. The interpolators measure time intervals with a switched-frequency ring oscillator. The frequency switching is used as a mechanism to amplify the quantization error in the cyclic interpolator. A digital calibration scheme is used for radix extraction. The interpolators' worst case INL is ±4.5 ps. Due to the interpolator's INL, the TDC's RMS precision is about 4.2 ps, while the worst and best case single-shot precisions are 5.5 ps and 1.7 ps, respectively. The overall accuracy of the TDC is better than 5 ps in a temperature range of -30 C to 70 C. The TDC is designed in 0.35 μm CMOS technology and consumes 80 mW at 0.8 MHz measurement rate.