Abstract: An analog-to-digital (A/D) converter circuit in which an extracted envelope of an input signal is used as a reference signal on an analog-to-digital converter, providing a wide dynamic range while avoiding the need for automatic gain control. Specifically, the A/D converter circuit includes a rectifier for rectifying an analog input signal, a filter for filtering a rectified analog signal output from the rectifier to provide a signal envelope used as the reference signal, and an analog-to-digital converter for converting the analog input signal into a digital signal within a range, the range being dynamically set in accordance with the reference signal provided by the filter.