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

Full-Scale (Offset) Error111ACTUALFull-ScaleError110OUTPUT CODE101100011010IDEAL0010001/8 1/4 3/8 1/2 5/8 3/4 7/8 FSANALOG INPUT (V)10NCG 9/99In an ideal A/D converter, the output code transition to full scale just barely occurs when theinput voltage equals G * V REF * (2 n - 1.5) / 2 n , where “G” is the gain of the converter (usually“1”), V REF is the ADC reference voltage and “n” is the resolution (number of output bits) of theADC.In an actual ADC the full-scale analog input causing this transition may differ somewhat fromthis ideal value. Full Scale Error is the error in the actual full-scale output transition point fromthe ideal value. Part of this error will be due to offset voltage and the rest will be due to anerror in the slope of the transfer function. Full Scale Error may be expressed in LSBs or as apercentage of the full-scale voltage.Full Scale Error is sometimes called Full Scale Offset Error and is expressed in LSBs, Volts oras a percentage of ideal full scale input.Top Offset is yet another type of full scale error, defined as the difference between the positivereference voltage and the input voltage that just causes the output code to transition to fullscale plus 1.5 LSB, or V FS :E OT = V FT +1.5 LSB - V REF = V FS – V REFwhere E OT is the Top Offset voltageV FT is the input voltage causing the full-scale transitionV REF is the ADC reference voltageV FS is 1.5 LSB above V FT .Top Offset is a very unusual specification.ABCs of ADCs - Rev 3, June 2006Authored by: Nicholas “Nick” Gray10Copyright © 2003, 2004, 2006 National SemiconductorCorporationAll rights reserved