ADC (Analog to Digital Converter)
Introduction:
Design and test a flash analog-to-digital converter circuit using op amps. The input to the circuit will be an analog voltage ranging from 0 to 1V. The output will be a 3-bit digital number. The output should range from 000 when the input is 0 V to 111 (7) when the input is 1V. Note that each digital number will correspond to a range of analog inputs. Also note that some digital logic will be required in order to take the output of the flash converter and "decode" it into the correct digital numbers.
Circuit Design:
We use the design from the website. We need 7 different outputs to form an 8 different output values (0 – 7) circuit, and we use the concept of the voltage divider (similar on the website). Since we need 8 different voltages for 8 different outputs value, we need 8 1k ohms to divide the reference voltage to seven different voltages: 0.125V, 0.25V, 0.375V, 0.5V, 0.625V, 0.75V, 0.875V, from output 7(the lowest value) to output 1(the highest value). The input voltage is connected to positive inputs (non-inverting inputs) of all 7 comparators. When the input voltage increases, the output of each particular comparator will be determined by the reference voltage (whether to go high or not). The output will go high from output 7 to output 1 one by one if we increase the input voltage from 0 to 1 V. When the voltage is higher than the reference voltage of the 1st comparator (highest output), all the output should go high and the (logic) output should be 111 (in binary value) which is 7 in decimal value. The outputs of 7 comparators are connected to a decoder which decodes the output from comparators to a 3 bit binary value. The design of the “decoder” is by connecting the outputs of the comparators to digital logic circuit to have three outputs of X (the most significant bit), Y (2nd most significant bit), Z (least significant bit) to represent three bits of binary value. The implement of the logic is showed below.
|
out 1 |
out 2 |
out 3 |
out 4 |
out 5 |
out 6 |
out 7 |
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X |
Y |
Z |
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0 |
0 |
0 |
0 |
0 |
0 |
0 |
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0 |
0 |
0 |
|
0 |
0 |
0 |
0 |
0 |
0 |
1 |
|
0 |
0 |
1 |
|
0 |
0 |
0 |
0 |
0 |
1 |
1 |
|
0 |
1 |
0 |
|
0 |
0 |
0 |
0 |
1 |
1 |
1 |
|
0 |
1 |
1 |
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0 |
0 |
0 |
1 |
1 |
1 |
1 |
|
1 |
0 |
0 |
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0 |
0 |
1 |
1 |
1 |
1 |
1 |
|
1 |
0 |
1 |
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0 |
1 |
1 |
1 |
1 |
1 |
1 |
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1 |
1 |
0 |
|
1 |
1 |
1 |
1 |
1 |
1 |
1 |
|
1 |
1 |
1 |
And the output logic is below:
X = out 4
Y = out 2 · out 6 + out 4’ · out 6
Z = out 1 + out 2’ · out 3 + out 4’ · out 5 + out 6’ · out 7
Conclusion:
In this project, the concept is quite simple, but we need to measure the resistance for each resistor very accurately because each of them can affect the reference voltage for each comparator, and this part takes the longest time to complete the project.
Circuit
Diagram:
