the raw isig is not a direct measure of the current. If it was a single measure they would need a 24 or 32 bits ADC and at that level of signal I thought it would probably be overkill (I can't picture 12 or 16db of dynamic range in the interstitial glucose signal - maybe I am wrong - just a gut feeling). That gut feeling made me dig a bit: they subcontracted an asic in their first generation products but now seem to have an in-house team - this dexcom patent is interesting http://www.google.com/patents/US20140266776
, especially the "adjustable integration window" part. No definite answer, but pointers on how they do it at least. And they could still be using a 32 bits ADC in their asic knowing it is overkill simply because a drop-in component is available. Or they could be using 8 bits and returning the sum of many measures.