Two things together: the bit count N and the reference voltage Vref. The ADC divides the span from 0 to Vref into 2^N levels, so the smallest distinguishable step, one LSB, is Vref / 2^N volts. A 12-bit ADC with a 3.3 V reference has 4096 levels and an LSB of about 0.806 mV. So "12-bit" alone doesn't tell you the voltage resolution; you need Vref too, the same 12 bits over a 1.0 V reference gives a finer 0.244 mV LSB but a smaller measurable range. Crucially, nominal resolution isn't the same as usable resolution: noise on Vref and on the input, plus the ADC's own non-idealities, mean the effective number of noise-free bits is typically several less than N. So real resolution is set by N, Vref, and the noise floor, and a quiet, accurate reference is as important as the bit count.
Embedded Systems Fundamentals · Interview question
What determines the resolution of an ADC reading in real volts?
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From the lesson
ADC & DAC
Bridging the analog and digital worlds: how an ADC quantizes a voltage to an N-bit count (and a DAC reverses it), the count↔voltage math, and why Vref and Nyquist set your limits.