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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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• • • • •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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Mixed-Signal Device

May be Considered to be aDivider
– 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. Manyof 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 ADChas 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 (gainfactor). 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.

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PLEASE NOTE: The discussion here assumes an ADC with a binary output. Some of the statements here would be modifiedslightly for Offset Binary or 2’s Complement outputs.

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

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What, Exactly, Does An Analogto-Digital Converter Do?
• • For a 3-bit ADC, there are 8 possible output codes. In this example, if the input voltage is 5.5V and the reference is 8V,then the Analog output will be 101. Input More bits give better resolution and smaller steps. A lower reference voltage gives smaller steps, but can be at the expense of noise.
+VCC VREF (8V)

A/D
Converter

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0V < 000 < 1V 1V < 001 < 2V 2V < 010 < 3V 3V < 011 < 4V 4V < 100 < 5V 5V < 101 < 6V 6V < 110 < 7V 7V < 111 < 8V

GND

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Here is an example of a 3-bit A/D converter.Because it has 3 bits, there are 23 = 8 possible output codes. The difference between each output code is VREF / 23. Assuming that the output response has no errors, every time you increase the voltage at the input by 1 Volt, the output code will increase by one bit. This means, in this example, that the least 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 because we 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 is a common way for our customers to...

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