Hi @xlev
The ADC voltage reference on the Spark core is a filtered version of the 3.3V supply. If you are using USB or VIN to power the core, then this 3.3V comes from the on-board 3.3V regulator. If you are supplying your own 3.3V source, the voltage reference will only be as accurate as your supply.
If you add an external 1.2V reference IC like an LM185 series part, you can measure that on one ADC pin and measure your signal on another and use the ratio of the two to determine a more accurate value.