A photonic analog-to-digital converter (ADC) scheme based on multi-wavelength sampling and balanced detection is proposed and experimentally demonstrated. In the approach, a power-weighted multi-wavelength pulsed source is employed to implement sampling, a dispersion element is used to realize temporal walk-off of multi-wavelength pulses, and a thresholding module is applied to implement serial thresholding. The principles of quantization and encoding are similar to that of an electronic flash ADC, but the serial implementation in the given approach avoids the use of large numbers of comparators. A proof-of-concept experiment with three wavelengths is successfully demonstrated. We also discuss the feasibility of the approach and the design of non-uniform quantization by properly setting the power ratio of the multi-wavelength pulses.