In this work the main failure mechanism of irradiated charge redistribution Successive Approximation Register (SAR) Analog-to-Digital Converters (ADCs) is studied. Results of a gamma radiation experiment on two identical 130nm, 8-bit SAR ADCs (implemented in the same die), operating with distinct sampling rates, show that the degradation severity depends on the sampling frequency. The converter operating at lower frequency fails at lower accumulated dose, while increasing the sampling frequency increases the converter robustness to radiation. A SPICE model of a SAR ADC is used to simulate radiation induced leakage effects, considering the same technology node and operating conditions of the tested converters. Simulation results show a very good agreement with experimental data, allowing to explain the main failure mechanism, which is related to leakage in switches connected to the programmable capacitor array of the internal DAC of the converter.