MadSci Network: Engineering

Re: How to build and design a circuit for analog to digital conversion?

Date: Thu Jan 7 10:09:36 1999
Posted By: Lawrence Skarin, Faculty, Electrical Engineering, Monroe Community College
Area of science: Engineering
ID: 913121522.Eg

Hello, Mr. Yee.  These days, it is much simpler to buy an analog to digital
converter (ADC) integrated circuit, and apply it per manufacturer application 
notes, than to build one.

An ADC represents on input analog voltage by a digital output bit pattern.
An ADC does this by "sampling" the input during a short time interval,
and "quantizing"  choosing the bit pattern the voltage fits into.  Here's
a simple example: an ADC with 0-8 vDC input, and a 3-bit output.  This
means we can have 2^3 quantization levels -- eight of them.  So, here's
a table indicating what input ranges give what output bit patterns:

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

This ADC has 3-bit resolution, which can only represent to the nearest
volt.  4 bits would allow representation to the nearest 0.5 volts. because
you would then have 2^4 quantization levels.  You choose ADCs based
on what resolution your application requires.  This is not the only
consideration.  How many samples per second needs is another big one.

How the input voltage is evaluated comprises several methods like "flash,"
single slope integration, successive approximation, and dual-slope 

How the output bit pattern is presented also varies.  It could be on parallel
wires, or on one wire in serial form.  What I'm saying is there are so many 
flavors of ADCs, you want to read a good tutorial on them so you can
choose intelligently.  Either find an electronics text whose index devotes
pages to the kinds of ADCs in the previous paragraph, and/or try this link:

Good luck!

Larry Skarin

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