ABCs_of_ADCs - ABCs of ADCs Analog-to-Digital Converter...

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ABCs of ADCs - Rev 3, June 2006 Authored by: Nicholas “Nick” Gray Copyright 2003, 2004, 2006 National Semiconductor Corporation All rights reserved 1 ABCs of ADCs Analog-to-Digital Converter Basics Nicholas Gray Data Conversion Systems Staff Applications Engineer November 24, 2003 Corrected August 13, 2004 Additional Corrections June 27, 2006
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ABCs of ADCs - Rev 3, June 2006 Authored by: Nicholas “Nick” Gray Copyright 2003, 2004, 2006 National Semiconductor Corporation All rights reserved 2 2 Agenda - ABCs of ADCs What’s an ADC? Review of Definitions Sources of Distortion and Noise Common Design Mistakes High Speed ADCs at National
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ABCs of ADCs - Rev 3, June 2006 Authored by: Nicholas “Nick” Gray Copyright 2003, 2004, 2006 National Semiconductor Corporation All rights reserved 3 3 October 2001 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) Because the Analog-to-Digital Converter (A/D Converter or ADC) has both analog and digital functions, 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 that provides 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 the true 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 is compared. The digital output word tells us what fraction of the reference voltage or current is the 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 this formula before, you probably did not see the “G” term (gain factor). This is because we generally consider this to be unity. However, National Semiconductor has introduced ADCs with 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 the statements here would be modified slightly for Offset Binary or 2’s Complement outputs.
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ABCs of ADCs - Rev 3, June 2006 Authored by: Nicholas “Nick” Gray Copyright 2003, 2004, 2006 National Semiconductor Corporation All rights reserved 4 Here is an example of a 3-bit A/D converter. Because it has 3 bits, there are 2 3 = 8 possible output codes. The difference between each output code is V REF / 2 3 .
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