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In this paper, a time-based analog-to-digital converter (ADC) with first order noise shaping property is presented. The ADC consists of a pulse-width modulator (PWM) and a time-to-digital converter (TDC) based on a gated-ring oscillator. A prototype is designed in 0.18μm CMOS and simulated to verify the concept. The ADC shows high linearity despite the open loop architecture since the linearity does not depend on the frequency tuning characteristic of oscillators unlike conventional noise shaping time-based ADCs. With the sampling frequency of 10MHz, the prototype achieves spurious-free dynamic range (SFDR) and signal-to-noise-and-distortion ratio (SNDR) of 66dB and 56dB, respectively, when the signal bandwidth is 500kHz.