A robust, self-consistent, event-triggered incremental 13-bit A/D converter for space spectroscopy (ORION ADC) has been developed and characterized. The main application of the proposed circuit is the detection of X and γ rays in space within the THESEUS exploration mission, whose aim is a deeper investigation in high-energy transient phenomena over the complete universe history. The first fabricated prototype, in CMOS 0.35 �m technology includes the full ADC mixed signal architecture plus all ancillary but fundamental blocks for its successful characterization, including voltage reference and input signal buffers. The circuit is based on a second order sigma-delta modulator architecture which can be reset before each conversion to avoid hysteretic behavior over different samples processing. This requirement is mandatory in spectroscopic applications. Since the converter will be employed in a dual-mode front-to-back-end circuit for X and γ ray detection, it must exhibit maximum flexibility and re-configurability. Several internal analog and digital test points can be interrogated by the characterization setup and a dedicated internal logic can carry out digital decimation and filtering operations, as well as control the ADC state machine and accuracy/conversion time trade-off mode preference. The ORION ADC is one of the key-modules of the core ASIC of the THESEUS mission spectroscopy ORION, thus particular focus has been given to reliability and robustness. Finally, event-based conversion leads to power consumption reduction, while guaranteeing very short latency in conversion time.