Version 1.0, August 31, 2001, Copyright, Hugh Jack 1993-2001

29.1.1 Analog To Digital Conversions

 

· When there are analog values outside a computer, and we plan to read these to digital values, there are a variety of factors to consider,

  1. - when the sample is requested, a short period of time passes before the final sample value is obtained.
  2. - the sample value is `frozen' after a sample interval.
  3. - after the sample is taken, the system may change
  4. - sample values can be very sensitive to noise
  5. - the continuous values of the signal loose some accuracy when conversion to a digital number

 

· Consider the conversion process pictured below,

 

 

· Once this signal is processes through a typical A/D converter we get the following relations (these may vary slightly for different types of A/D converters).

 

 

· Consider the example,

 

 

· In most applications a sample is taken at regular intervals, with a period of `T' seconds.

 

· In practice the sample interval is kept as small as possible. (i.e., tau << T)

 

· If we are sampling a periodic signal that changes near or faster that the sampling rate, there is a chance that we will get a signal that appears chaotic, or seems to be a lower frequency. This phenomenon is known as aliasing.

 

· Quite often an A/D converter will multiplex between various inputs. As it switches the voltage will be sampled by a `sample and hold circuit'. This will then be converted to a digital value. The sample and hold circuits can be used before the multiplexer to collect data values at the same instant in time.

 

· A simple type of A/D converter is shown below. It is known as a successive approximation type.