How do you calculate the voltage represented by one LSB, and why does it matter?

The least significant bit (LSB) voltage is the smallest voltage change the ADC can distinguish — the quantization step size:

LSB = V_REF / 2^n

where V_REF is the reference voltage and n is the resolution in bits. For a 12-bit ADC with a 3.3V reference: LSB = 3.3 / 4096 = 0.806 mV. To convert a raw ADC reading back to voltage: V = raw_value * V_REF / 2^n. For example, a raw reading of 2048 corresponds to 2048 * 3.3 / 4096 = 1.65V — exactly half the reference, as expected for the midpoint code.

The LSB voltage matters because it defines the quantization limit — any signal change smaller than 1 LSB is invisible to the converter. It directly determines whether the ADC has sufficient precision for your measurement. Consider a temperature sensor that outputs 10 mV per degree C. If you need 0.1-degree resolution, you need an LSB of 1 mV or less. With a 3.3V reference, a 12-bit ADC (0.806 mV LSB) barely meets this requirement, while a 10-bit ADC (3.22 mV LSB) does not. The options are: use a higher-resolution ADC, add an analog gain stage to amplify the signal before conversion (an op-amp with gain of 10x would make the signal 100 mV/degC, well above the LSB), or use oversampling to gain effective bits in software.

A common interview trap: candidates forget that the LSB calculation assumes a perfect ADC. Real ADCs have noise, offset error, gain error, and nonlinearity (INL/DNL) that degrade the effective resolution below the theoretical LSB. Always check the ENOB (Effective Number of Bits) specification to understand what resolution you actually get.