Understanding ENOB (Effective Number of Bits) in ADCs – A Practical Guide

When selecting or evaluating an analog-to-digital converter (ADC) for dynamic or AC signal measurements, ADC users often encounter a dizzying array of dynamic specifications: SNR, SINAD, THD, and more. But sometimes it can be hard to understand the impact that these specifications have on real world signal measurement. That is where the ENOB (Effective Number of Bits) Specification comes into play. ENOB takes into account key dynamic specifications and turns them into a specification that is easier to understand the affect on your real world signal measurement. Unlike the ideal resolution specification for your ADC (12 bit, 14 bit, 16 bit, etc), ENOB tells you how many bits of real or useful information your ADC is delivering in the presence of noise and distortion.

📘 What is ENOB?

ENOB (Effective Number of Bits) is a measure of an ADC's actual performance, indicating how many bits of the digital output are meaningful after accounting for noise and nonlinearity.

It answers the question:

“Out of my ADC’s 16 bits, how many are actually useful in real-world measurements?”

ENOB combines key dynamic error sources—noise and harmonic distortion—into a single, digestible metric.

🔣 ENOB Calculation Formula

ENOB is typically calculated from the SINAD (Signal-to-Noise and Distortion Ratio) of the ADC. And SINAD is a combination of the SNR (Signal-to-Noise Ratio) and THD (Total Harmonic Distortion) specifications. Confused yet? Here are some quick definitions of SNR, THD, and SINAD:

  • SNR (Signal-to-Noise Ratio):
    Ratio of the signal power to all non-harmonic noise (thermal noise, quantization noise, jitter, etc.). Does not include harmonic distortion.
  • THD (Total Harmonic Distortion):
    Ratio of the total harmonic distortion power (2nd, 3rd, etc.) to the signal power. Does not include random noise.
  • SINAD (Signal-to-Noise and Distortion Ratio):
    Ratio (Expressed in dB) of signal power to all other spectral components, including:
    • Noise (same as in SNR)
    • Harmonic distortion (same as in THD)

Note: if you want more information on these specs and the math behind them see link to Analog Devices app note at the end of this tutorial.

Example ENOB Calculation – Real-World ADC

A lot of ADCs on the market will include an ENOB specification in their datasheet, but not all so let's look at how to calculate ENOB from SINAD. Before we do, it is important to keep in mind that ADC datasheets will have a table or a plot or both of SINAD  specifications since it varies based on the input signal's frequency, the ADC's sampling rate, input signal amplitude, voltage reference specs, and more. If you have to do an ENOB calculation be sure to use the SINAD specifications that match your use case.

  • Let’s say a 16-bit ADC has a SINAD Specification of 85 dB under your operating conditions.
  • The formula: ENOB = SINAD - 1.76 / 6.02 dB
    • 1.76 accounts for quantization noise in an ideal ADC
    • 6.02 dB ≈ 20 × log₁₀(2) for each bit
  • Plugging in our example SINAD specification: ENOB = 85 - 1.76 / 6.03 = ~13.8 bits

So even though your ADC is 16-bit on paper, you're only getting ~13.8 bits of meaningful resolution. The rest is lost to noise and distortion. If you are evaluating two ADCs that have the same resolution and sample rate, but one is priced much higher than the other it is probably because the higher cost one has a better ENOB specification. 

🔬 ENOB beyond just the ADC

ENOB becomes critically important in applications where accuracy, not just resolution, affects system performance. And when we think of applications or circuits that use ADCs it is not just the ADC that can add noise and distortion to the signal. Fast sampling ADC circuits will have "Front End" designs made up of one or multiple op amps which have their own noise and harmonic distortion specifications. The circuit may employ an external voltage reference for the ADC which will have some type of noise specifications as well. That means when you want to understand the ENOB of an entire ADC circuit design you need to take into account all the noise and distortion contributors, not just the ADC's ENOB specification. 

🧠 Final Thoughts

ENOB is more than just a number—it’s a practical measure of analog-to-digital conversion fidelity. While datasheets may boast high bit counts, real-world performance hinges on how clean your signal is and how well your ADC preserves it.

Always check the ENOB under your target input frequency and amplitude, and don’t assume all bits are created equal!

Analog Devices App Note: How to Calculate ENOB for ADC Dynamic Performance Measurement

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