Why ADC Reference Voltage Directly Affects Measurement
When debugging ADC measurements, it is natural to focus on the input pin: sensor output, divider ratio, front-end filtering.
But the ADC result is not determined by input voltage alone. Reference voltage participates in every conversion.
A more accurate first model is:
ADC code ≈ Vin / Vref * full scale
The ADC reads the ratio between input voltage Vin and reference voltage Vref. If Vin changes, the result changes. If Vref changes, the result also changes.