Abstract: In one or more embodiments, an apparatus and method for processing an analog signal into a digital signal includes an input current buffer circuit, a signal charge integration node, a dual function comparator, a step charge subtractor, a state latch, a coarse N-bit counter, an optional residue signal buffer and a residue signal M-bit time-to-digital (TDC) converter. The circuitry is free running, meaning that it is never reset. Instead, what is tracked for each frame is how much additional charge has been accumulated since the end of the previous integration period. Between each frame, the state of the counter and the amount of charge residing in the integration node are recorded. This information from the beginning and end of a given frame is differenced and to this is added the amount of charge indicated by the number of times the counter overflowed during the integration period.