Abc dos adc

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

ABCs of ADCs - Rev 3, June 2006 Authored by: Nicholas “Nick” Gray

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Copyright © 2003, 2004, 2006 National Semiconductor Corporation All rights reserved

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

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Copyright © 2003, 2004, 2006 National Semiconductor Corporation All rights reserved

What Is an ADC?
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Mixed-Signal Device
– Analog Input – Digital Output

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May be Considered to be a Divider
– Output says: Input is What Fraction of VREF? – Output = 2n x G x AIN / VREF • n = # of Output Bits (Resolution) • G = Gain Factor (usually “1”) • AIN = Analog Input Voltage (or Current) • VREF (IREF)= Reference Voltage (or Current)
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October 2001

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