ENOB relates to measured ADC performance by equating the measured SNR to that of an ideal N-bit ADC. Which statement best expresses ENOB?

• A. ENOB equals the number of bits of an ideal ADC that would yield the same SNR as the measured system.
• B. ENOB is the actual number of bits in the ADC’s architecture.
• C. ENOB is the ratio of distortion to signal energy.
• D. ENOB determines the input bandwidth.

Answer: (A)

ENOB expresses the number of bits of an ideal ADC that would give the same signal-to-noise-plus-distortion level as the real converter. In practice you measure the ADC’s SNR for a known input (often a full-scale sine) and then map that SNR to an equivalent bit count using the relation for an ideal N-bit ADC: ENOB ≈ (SNR_meas_dB − 1.76) / 6.02. This shows the effective resolution after accounting for all noise and distortion, which may be higher or lower than the physical bit depth and can be a non-integer. ENOB is not a measure of actual architecture bits, nor of distortion-to-signal energy alone, nor of input bandwidth.