We present the design of a wide dynamic-range CMOS A/D interface circuit for ionization chambers, able to digitize input charge of both polarities over six decades, to operate in a radioactive environment, and foreseen working without calibration. The circuit is based on a current-to-frequency conversion, where the converting circuit reconfigures itself depending on the input level to provide an output code every 40 μs. Simulation results of its implementation in a 0.25 μm CMOS are presented, showing its operation over a 120dB dynamic range. The simulated circuit consumes less than 33 mW from a 2.5-V supply. © 2011 IEEE.