ENOB vs SINAD vs SNR Explained (for Analog Engineers)

In analog and mixed-signal design, ADC performance is often described using terms like SNR, SINAD, and ENOB. These parameters frequently appear in interviews, datasheets, and real-world design discussions. Let’s break them down in simple terms with formulas and examples.

What is SNR (Signal-to-Noise Ratio)?

Definition: Ratio of signal power to noise power, usually expressed in decibels (dB).

Formula: SNR = 10 × log10 (Signal Power / Noise Power)

Example: If an ADC outputs a sine wave with noise floor 60 dB below it, SNR = 60 dB. This means the signal is 1000× stronger than noise.

What is SINAD (Signal-to-Noise and Distortion)?

Definition: Ratio of signal power to the combined power of noise + distortion components.

Formula: SINAD = 10 × log10 (Signal Power / (Noise Power + Distortion Power))

Example: If SNR = 60 dB but distortion adds 5 dB more error, then SINAD ≈ 55 dB. This is a more realistic measure of ADC quality.

What is ENOB (Effective Number of Bits)?

Definition: The number of bits of an ideal ADC that would provide the same resolution as a real ADC with a given SINAD.

Formula: ENOB = (SINAD – 1.76) / 6.02

Example: If an ADC has SINAD = 50 dB:
ENOB = (50 – 1.76) / 6.02 ≈ 8 bits.

Comparison Table

Why These Matter in Interviews

• Common interview question: difference between SNR and SINAD.
• ENOB links ADC datasheet values to real usable resolution.
• Quick ENOB calculation shows practical understanding.

Sample Interview Question

If an ADC has SINAD = 50 dB, what is ENOB?

Solution: ENOB = (50 – 1.76) / 6.02 ≈ 8 bits.

Conclusion

SNR measures pure noise, SINAD includes distortion, and ENOB converts SINAD into equivalent resolution bits. These parameters are critical for understanding and comparing ADCs.