page 19
Vref
|
Vin |
R/2 |
|
R |
|
R |
Decimal to binary |
converter |
binary out
R
R
R
R
R
R/2
•These converters are very fast, but they are hard to build for high resolutions.
•The conversion rates for these devices are limited by inherent capacitance, and transistor switching times.
4.2.2 Digital to Analog Conversion
•After we have used a controller equation to estimate a value to put into our process, we must convert this from a digital value in the computers memory, to a physical voltage.
•This voltage is typically limited to 20mA in most computer board, and drawing near this current
page 20
reduces accuracy and life of the board.
• A simple circuit is shown below for a simple digital to analog converter.
MSB bit 3
bit 2
Computer
bit 1
LSB bit 0
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5KΩ |
10KΩ |
V– |
|
- |
Vss |
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|
20KΩ |
V |
+ |
+ |
+ |
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0 |
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|
Vo |
40KΩ |
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- |
80KΩ |
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First we write the obvious,
V + = 0 = V–
Next, sum the currents into the inverting input as a function of the output voltage and the input voltages from the computer,
|
Vb3 |
Vb2 |
Vb1 |
Vb0 |
Vo |
10--------------KΩ |
+ 20--------------KΩ |
+ 40--------------KΩ |
+ 80--------------KΩ |
= 5-----------KΩ |
|
|
Vo |
= 0.5Vb3 |
+ 0.25Vb2 + 0.125Vb1 + 0.0625Vb0 |
||
Consider an example where the binary output is 1110, with 5V for on, |
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|
Vo |
= 0.5( 5V) + 0.25( 5V) + 0.125( 5V) + 0.625( 0V) = 4.375V |
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•The calculations for the A/D converter resolution and accuracy still apply.
•Consider the example below,