A ΔΣ all-digital delay-locked loop (ADDLL) is proposed to realize a PVT-insensitive time-to-digital converter (TDC) with enhanced linearity in an all-digital phase-locked loop (ADPLL). With the proposed TDC, poor timing resolution and nonlinearity problems are mitigated, enabling a low cost, low comparison frequency TDC design without using the advanced CMOS technology. A novel digitally-controlled delay line (DCDL) is proposed to ensure monotonous and linear mapping between a digital control word and a total time delay. A phase error compensator (PEC) is employed to calibrate periodic phase error of the proposed TDC. A 2.5GHz ADPLL is designed in 0.18?m CMOS. Simulation results show that the proposed method effectively reduces fractional spurs caused by the TDC.