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

DNL1111101010.3 LSB;DNL = –0.7IdealActualDigital Output1000110101.2 LSB;DNL = +0.22.2 LSB;DNL = +1.2Missing Code (100)0010001.0 LSB;DNL = 01.3 LSB;DNL = +0.3250 500 750 1000 1250 1500 1750 2000INPUT VOLTAGE (mV)V REF= 2.0V13In an ideal converter, the code-to-code transition points are exactly 1 LSB apart. In an 8-bitADC, for example, these changes are separated from each other by 1 LSB, or 1 / 256 of fullscale.The difference between the ideal 1 LSB and the worst case actual input voltage changebetween output code transitions is called Differential Non-Linearity.DNL can be illustrated using the transfer function of a three-bit DAC shown above. Each inputstep should be precisely 1 / 8 of full-scale. In the example above, the first code transition (from000 to 001) is caused by an input change of FS / 8 (250mV for the 2 Volt reference exampleshown here), where FS is the full-scale input. This is exactly as it should be. The secondtransition, from 001 to 010, has an input change that is 1.2 LSB, so is too large by 0.2 LSB.The input change for the third transition is exactly the right size. The digital output remainsconstant when the input voltage changes from 1000mV to beyond 1500mV and the code 101can never appear at the output. It is missing. To avoid missing codes in the transfer function,DNL should be greater (more positive) than -1.0 LSB.DNL indicates the deviation from the ideal 1 LSB step size of the analog input signalcorresponding to a code-to-code increment. DNL, a static specification, relates to SNR, adynamic specification. However, noise performance can not be predicted from DNLperformance, except to say that SNR tends to become worse as DNL departs from zero.ABCs of ADCs - Rev 3, June 2006Authored by: Nicholas “Nick” Gray13Copyright © 2003, 2004, 2006 National SemiconductorCorporationAll rights reserved