Effective number of bits
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Effective number of bits (ENOB) is a measure of the real dynamic range of an analog-to-digital converter (ADC), digital-to-analog converter (DAC), or associated circuitry. Although the resolution of a converter may be specified by the number of bits used to represent the analog value, real circuits however are imperfect and introduce additional noise and distortion. Those imperfections reduce the number of bits of accuracy. The ENOB describes the effective resolution of a real converter in terms of the number of bits an ideal converter with the same resolution would have.[1]
ENOB is also used as a quality measure for other blocks such as sample-and-hold amplifiers. Thus analog blocks may be included in signal-chain calculations. The total ENOB of a chain of blocks is usually less than the ENOB of the worst block.
Definition
An often used definition for ENOB is[2]
where
- ENOB is given in bits
- SINAD (signal, noise, and distortion) is a power ratio indicating the quality of the signal in dB.
- the 6.02 term in the divisor converts decibels (a log10 representation) to bits (a log2 representation),[note 1]
- the 1.76 term comes from quantization error in an ideal ADC.[4]
This definition compares the SINAD of an ideal converter of ENOB bits with the SINAD of the converter being tested. The ENOB may be fractional. So while an ADC may use 12 bits, its ENOB may only be 9.5, corresponding to a hypothetical ideal converter with 9.5 bits.
Effective resolution bandwidth
Notes
- ↑ .
References
- ↑ "Understanding ADC specifications" (in en). 2015-03-04. https://community.nxp.com/t5/Kinetis-Microcontrollers/Understanding-ADC-specifications/ta-p/1110050.
- ↑ Kester 2009, p. 5, Equation 1.
- ↑ Eq. 2.8 in Geerts, Yves; Steyaert, Michiel; Sansen, Willy M. C. (2002). Design of multi-bit delta-sigma A/D converters. Springer. ISBN 9781402070785.
- ↑ .[3]
- Gielen, Georges (2006). Analog Building Blocks for Signal Processing. Leuven: KULeuven-ESAT-MICAS.
- Kester, Walt (2009), Understand SINAD, ENOB, SNR, THD, THD + N, and SFDR so You Don't Get Lost in the Noise Floor, Tutorial, Analog Devices, MT-003, https://www.analog.com/media/en/training-seminars/tutorials/MT-003.pdf
- Maxim (December 17, 2001), Glossary of Frequently Used High-Speed Data Converter Terms, Application Note, Maxim, 740, http://www.maxim-ic.com/appnotes.cfm/appnote_number/740/
External links
- Video tutorial on ENOB from Texas Instruments
- The Effective Number of Bits (ENOB) (PDF) April 2011 Rohde & Schwarz - This application note explains how to measure the oscilloscope ENOB.
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