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

What, Exactly, Does An Analogto-DigitalConverter Do?• For a 3-bit ADC, there are 8possible output codes.• In this example, if the inputvoltage is 5.5V and thereference is 8V, then theoutput will be 101.• More bits give betterresolution and smaller steps.• A lower reference voltagegives smaller steps, but canbe at the expense of noise.AnalogInput+V CC V REF (8V)A/DConverterGND0V < 000 < 1V1V < 001 < 2V2V < 010 < 3V3V < 011 < 4V4V < 100 < 5V5V < 101 < 6V6V < 110 < 7V7V < 111 < 8V4Here is an example of a 3-bit A/D converter. Because it has 3 bits, there are 2 3 = 8 possibleoutput codes. The difference between each output code is V REF / 2 3 .Assuming that the output response has no errors, every time you increase the voltage at theinput by 1 Volt, the output code will increase by one bit. This means, in this example, that theleast significant bit (LSB) represents 1 Volt, which is the smallest increment that this convertercan resolve. For this reason, we can say that the resolution of this converter is 1.0V becausewe can resolve voltages as small as a volt. Resolution may also be stated in bits.Note that if you reduce the reference voltage to 0.8V, the LSB would then represent 100mV,allowing you to measure a smaller range of voltages (0 to 0.8V) with greater accuracy. This isa common way for our customers to get better precision from a converter without buying amore expensive, higher resolution converter.The Resolution of an A/D converter is the number of output bits it has (3 bits, in this example).Resolution may also be defined as the size of the LSB (Least Significant Bit) or one count (1Volt, in this example).ABCs of ADCs - Rev 3, June 2006Authored by: Nicholas “Nick” Gray4Copyright © 2003, 2004, 2006 National SemiconductorCorporationAll rights reserved