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In this work, the design and study of a complete system for the readout of photocurrents in high dynamic range applications is presented. The system implements a novel readout circuit, with a high input dynamic range, based on the Fractional Packet Counting concept. The proposed current-to-digital conversion scheme works in two steps: first of all a switched integrator is used to obtain a coarse conversion of the input signal by counting the number of times it is reset during the signal integration period. This method estimates the number of “charge packets” read by the system. For added accuracy, a second step uses a conventional ADC to convert the last incomplete charge packet at the end of the constant signal integration time, providing a “fine” readout. The system includes: circuits for biasing the device under test; an integrated heater coupled with a set of temperature sensors for temperature control of the device under test; a LED with 5-bit intensity control for the operation of the devices under illumination; a microcontroller for autonomous operation. Preliminary measurements performed on the fabricated board confirmed the correctness of the working principle. In particular the measured dynamic range has been found to be larger than 150 dB with the input current range extending from 32fA up to 2μA.