Lesson 22: Analog to Digital Conversion Objectives



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EC312 Lesson 22 Analog to Digital Course Notes

4.1 Sampling
This is a process of inspecting the value (voltage) of an analog signal at regular time intervals. The 
time between samples is referred to as the sample period (
T
, in seconds), and the number of samples taken per 
second is referred to as the sample frequency (
f
s
, in samples/second or Hz). Basically, sampling is taking snap-shot 
values of the analog signal so that you have an accurate representation of how the analog signal is changing over 
time.
The receiver must convert the bits it receives into sample values, and then recreate what it thinks the analog signal 
looks like from the samples alone. As you might deduce from the figure below, when the samples are closer together 
(smaller sample period, which means higher sample frequency), the analog signal is more accurately represented. 
Note that with the lower sample rates, some of the fluctuations in the analog signal have no samples on them, so the 
samples are not a good representation of the analog signal. How high does our sampling frequency 
f
s
need to be in 
order to accurately represent the signal? That is, what is the minimum sample frequency for the A/D to work 
properly? 
We could consider taking just a few samples (i.e., using a low sampling rate), which means less information to 
transmit to the receiver. But if we choose that option, when we reconstruct the signal, it will likely be a terrible 
representation of the original. The low sampling rate will only work well for very slowly changing (low frequency) 
signals. Alternatively, we could choose the highest possible sampling rate known to man, to ensure that we can 
accurately capture even very fast signal fluctuations. But the higher the sampling rate, the higher the cost of the 
equipment and more information must be transmitted. In addition, if we decide to record the communications our 
saved files will be unnecessarily enormous. 


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But what is “low” and what is “high”? In other words, how exactly do I go about choosing my sampling rate? In 
order to accurately reconstruct an analog signal from its samples, one must sample 
faster
than the 

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