Aspects of the disclosure provide a method for calibrating pulse-width based measurement. The method includes generating a sloped time varying signal having a voltage level that varies with time, and generating a calibration pulse having a first calibration edge and a second calibration edge. The first calibration edge corresponds to a first calibration time that the sloped time varying signal has a first reference voltage, and the second calibration edge corresponds to a second calibration time that the sloped time varying signal has a second reference voltage. Further, the method includes modifying a parameter that governs a slope of the sloped time varying signal to calibrate the slope, such that a calibration pulse width of the calibration pulse is substantially equivalent to a reference time length. Then, the method includes performing pulse-width based measurement based on the calibrated slope.