Effective Number of Bits

Formula

Description

The effective number of bits measures the real-world performance of an ADC, accounting for all noise and distortion sources. An ideal N-bit ADC has SINAD = 6.02N + 1.76 dB. Working backward from a measured SINAD gives the ENOB, which is always less than the nominal bit count. For example, a 16-bit ADC might achieve only 13.5 ENOB at high input frequencies due to jitter, INL, and noise. ENOB is the single most important figure of merit for ADC dynamic performance and determines the actual useful resolution of the conversion.

Variables

• ENOB — Effective number of bits (dimensionless)
• SINAD — Signal-to-noise-and-distortion ratio (dB)

Practical Notes

The 6.02 comes from 20×log₁₀(2) and the 1.76 from 10×log₁₀(3/2) for a full-scale sine wave. ENOB decreases with input frequency due to aperture jitter (ENOB_jitter ≈ −20×log₁₀(2πf×tj)/6.02). A 1 ps jitter clock limits ENOB to about 11.1 bits at 100 MHz input. Always check ENOB at your actual signal frequency, not just DC.