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

What Is an ADC?• Mixed-Signal Device– Analog Input– Digital Output• May be Considered to be a Divider– Output says: Input is What Fraction of V REF ?– Output = 2 n x G x A IN / V REF• n = # of Output Bits (Resolution)• G = Gain Factor (usually “1”)• A IN = Analog Input Voltage (or Current)• V REF (I REF )= Reference Voltage (or Current)3October 2001Because the Analog-to-Digital Converter (A/D Converter or ADC) has both analog and digitalfunctions, it is a mixed-signal device. Many of us consider the ADC to be a mysterious device.It can, however, be considered very simply to be the instrument that it is: a device thatprovides an output that digitally represents the input voltage or current level.Notice I said voltage or current. Most ADCs convert an input voltage to a digital word, but thetrue definition of an ADC does include the possibility of an input current.An ADC has an analog reference voltage or current against which the analog input iscompared. The digital output word tells us what fraction of the reference voltage or current isthe input voltage or current. So, basically, the ADC is a divider.The Input/Output transfer function is given by the formula indicated here. If you have seen thisformula before, you probably did not see the “G” term (gain factor). This is because wegenerally consider this to be unity. However, National Semiconductor has introduced ADCswith other gain factors, so it is important to understand that this factor is present.____________________________________________________________________________PLEASE NOTE: The discussion here assumes an ADC with a binary output. Some of thestatements here would be modified slightly for Offset Binary or 2’s Complement outputs.ABCs of ADCs - Rev 3, June 2006Authored by: Nicholas “Nick” Gray3Copyright © 2003, 2004, 2006 National SemiconductorCorporationAll rights reserved