Abstract We propose the fast method of analog–digital conversion of optical signals by the use of waveguide ring resonators of micron size. The results of calculation are presented which allow optimization of the parameters of such optical analog–digital converter (ADC) for specific applications. Conversion operating speed of the ADC proposed is limited by the time of steady output signal establishing. For microresonator of 10 μm radius this time is about 10 ps. Analysis of the proposed optical method of analog–digital conversion shows that such optical ADC is capable to operate with the conversion frequency above 109 counts/s in a broad spectral range. Optical loss in resonator reduces its switching time. However, the switching contrast also decreases. To keep the switching contrast constant it is necessary to increase discretization step of the analog signal power.