Week #14: (12/8) ADCs & review.

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rjagodowski
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Joined: Fri Sep 04, 2015 6:59 pm

Week #14: (12/8) ADCs & review.

Post by rjagodowski »

This week we'll spend some time answering questions from Exam #1, then do a quick review of Analog to Digital Converters, specifically as to how they pertain to the Parallax Incubator circuit and the ADC0831.

This is explained in Chapter 6, but I wanted to summarize it here.

For the incubator circuit, the voltage on pin 3 of the ADC0831 is 0.7 volts. Note that this is the Vin(-) terminal, which effectively sets the minimum input voltage to 0.7 volts since the converted voltage must be positive, so Vin(+) - Vin(-) >= 0.

The 0.5V on pin 5 sets the range from 0.7V to 0.7V + 0.5V = 1.2V. Or, in terms of the LM34 temperature outputs, the ADC0831 is scaled to apply it's 8 bit output from 70 degrees to 120 degrees.

If this scaling was not not done, then the resolution of the converter would be the full range of input voltage (5V) divided by 255 (maximum binary number using 8 bits), or 19.6 mV. Since the LM34 outputs 10 mV/Degree, this implies that our Analog to Digital conversion accuracy would be limited to about 2 degree increments. Or put another way, the range of temperatures of 70 to 120 would only use the binary states between 36 (0.7v/19.6mV) and 61 (1.2V/19.6mV). Or, only 25 possible states would be used out of the 255 available. This isn't very efficient, and it reduces the accuracy.

By scaling the input by 0.7V and limiting the range to 0.5V, the full 255 binary combinations are now being used to represent 50 degrees total (from 70 to 120). Or, the resolution is 50 degrees/255 = 0.196 or about 0.2 degrees. By allowing ALL 255 states to be used within our conversion range, we have a more accurate system.
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