ABCs of ADCs - Analog-to-Digital Converter Basics (PDF)

ENOB - Effective Number OfBits• ENOB says that the ADC is equivalent tothis (ENOB) number of bits as far asSINAD is concerned. That is, a converterwith an ENOB of 7.0 has the same SINADas a theoretically perfect 7-bit converter.ENOB =SINAD - 1.766.0223Effective Bits (also called Effective Number Of Bits, or ENOB) is a specification that helps toquantify dynamic performance. ENOB says that the converter performs as if it were atheoretically perfect converter with a resolution of ENOB. That is, an ENOB of 7.5 bits saysthat converter performs, as far as SINAD is concerned, as if it were an ideal 7.5-Bit ADC.The ideal (perfect) ADC has absolutely no distortion and the only noise it exhibits isquantization noise, so SNR then equals SINAD. Since we know that SINAD for an ideal A/Dconverter is (6.02n + 1.76) dB, we can substitute “ENOB” for “n” and calculate:ENOB =where SINAD is expressed in dB.SINAD - 1.766.02So, the number of effective bits is another method of specifying SINAD. It says that theconverter is equivalent to a perfect ADC of this (ENOB) number of bits.ENOB degrades as frequency increases and as input level decreases for the same reasonsthat THD and SNR degrade with frequency increase and improves as input level increases.Remember, ENOB comes from SINAD, which comes from THD and SNR.ABCs of ADCs - Rev 3, June 2006Authored by: Nicholas “Nick” Gray23Copyright © 2003, 2004, 2006 National SemiconductorCorporationAll rights reserved