Question 1

What is the LSB (quantization step) of an ADC?

Accepted Answer

The least significant bit is the smallest voltage change the converter can resolve. For a reference voltage Vref split into 2^N levels it is LSB = Vref / 2^N. For example, a 12-bit ADC with Vref = 3.3 V has 4096 levels and an LSB of 3.3 / 4096 ≈ 805.7 µV.

Question 2

What is the ideal SNR of an N-bit ADC?

Accepted Answer

For a full-scale sine wave and ideal quantization the signal-to-noise ratio is SNR = 6.02·N + 1.76 dB. A 12-bit converter therefore has an ideal SNR of about 74.0 dB. The dynamic range, 6.02·N dB, is the same minus the 1.76 dB sine-wave term.

Question 3

What is ENOB and how is it calculated?

Accepted Answer

ENOB, the effective number of bits, converts a measured SNR (or SINAD) back into an equivalent ideal resolution: ENOB = (SNR_measured − 1.76) / 6.02. A real 12-bit ADC measuring 68 dB has ENOB = (68 − 1.76) / 6.02 ≈ 11.0 bits, so it behaves like an ideal 11-bit converter.

Question 4

Is the LSB Vref / 2^N or Vref / (2^N − 1)?

Accepted Answer

Both definitions are used. Vref / 2^N treats the full-scale range as one LSB per code transition and is the convention used for SNR and dynamic-range formulas. Vref / (2^N − 1) divides the range by the number of code transitions and is sometimes used to map the top code to exactly Vref. The difference is one part in 2^N, negligible for high resolutions but worth checking against your datasheet.

Question 5

Why is my real ADC worse than the ideal SNR?

Accepted Answer

The 6.02·N + 1.76 dB figure assumes perfect quantization of a full-scale sine wave. Real converters add thermal and reference noise, integral and differential nonlinearity (INL/DNL), clock jitter and limited bandwidth, all of which lower the measured SNR and therefore the ENOB below the nominal bit count.

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