Because the ADC measures the input as a ratio to Vref, the count is essentially V_in/Vref × 2^N. Every reading is scaled by Vref, so any error or noise in the reference translates directly into measurement error: if Vref drifts 1%, every reading is off 1%; if Vref has millivolts of ripple, that ripple appears in your counts. A common mistake is using the MCU's noisy digital 3.3 V supply (especially from a switching regulator) directly as Vref, it limits real accuracy to far below the ADC's nominal bit count no matter how many bits it has. For accurate absolute measurements you use a clean, stable, precise reference (a dedicated voltage-reference IC, or a filtered/separate analog supply), and you keep it well-decoupled. There's a useful exception: ratiometric measurements (like a resistive sensor in a divider powered from the same Vref) cancel the reference error, so absolute Vref accuracy matters less there. But in general, the reference quality caps your achievable accuracy.
Embedded Systems Fundamentals · Interview question
Why does the reference voltage matter so much for ADC accuracy?
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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.