What is ENOB, and why is it more meaningful than the advertised resolution?

ENOB (Effective Number of Bits) measures the ADC's actual resolution including all real-world error sources: thermal noise, quantization noise, harmonic distortion, INL (Integral Non-Linearity), DNL (Differential Non-Linearity), offset error, and gain error. It is calculated from the SINAD (Signal-to-Noise-and-Distortion ratio) of a full-scale sine wave test:

ENOB = (SINAD_dB - 1.76) / 6.02

A 12-bit ADC might advertise 12 bits of resolution but have an ENOB of only 10.5 bits, meaning the bottom 1.5 bits of every reading are dominated by noise and distortion. The advertised resolution (12 bits) is simply the number of digital output codes the converter produces — it says nothing about whether those codes are accurate. Two 12-bit ADCs from different manufacturers can have dramatically different ENOB values depending on their analog front-end design, reference quality, and layout.

ENOB matters for system design because it tells you the usable precision. If your application requires 12 bits of real accuracy (for example, a 0.05% measurement accuracy requirement translates to roughly 11 bits), a 12-bit ADC with 10.5 ENOB will not meet the spec — you need either a better-quality 12-bit ADC, a 14-bit ADC (whose ENOB might be 12+), or oversampling to recover the missing bits. Always check ENOB in the datasheet's electrical characteristics tables, not the marketing headline. Also note that ENOB degrades with input frequency (analog bandwidth limitations) and temperature — the datasheet ENOB is typically specified at a specific input frequency and temperature.