# Designing a DAC and ADC

Discussion in 'Homework Help' started by gusmas, Oct 21, 2008.

1. ### gusmas Thread Starter Active Member

Sep 27, 2008
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heya people its me again.... i got this project that requires me to design a ADC and then with then take the output of that ADC and feed it into a DAC and then i must see if there is any difference from my originally input voltage and my final voltage...

Aim for my project is: Design and build a complete data acquisition (DAQ) system using only discrete op-amps e.g. LM741. The system should include op-amp circuits for an analog-to-digital converter (ADC) and a digital-to-analog converter (DAC). The system will measure the accuracy of conversion where original input analog voltage is compared with the final output of the DAC. The closer these values are, the better the conversion process is. (NB! You will need to use a priority encoder

Specifications is:
(1) The DAC should accept an input sequence of four-digit binary numbers. A high level is a binary 1 and should be represented by +5V. A low level is a binary 0 and represented by 0V. The four-digit input value must be represented by four LEDs.
(2) The output of your DAC should represent the analogue value of the digital input e.g. the binary sequence 1111 should give you an output of +15V. Note the output of the DAC should be positive. How would you achieve this?

What i have got so far.....:

Ive decided that my input voltage is gona be 8volts.
I am gona use 15 OP-amps (4bit = 15 stages starting at 0000 as stage 1) and feed the ouputs to a Priority encoder to convert it to a Digital Signal
My Vref is going to be 16volts.

So have i started this project correctly??

2. ### RiJoRI Well-Known Member

Aug 15, 2007
536
29
While 15 op-amps may work, the cost would be prohibitive (think tens of thousands of them in a production environment).

Look into an R-2R ladder into ONE op-amp. Info is probably in the books here on-site.

--Rich

3. ### gusmas Thread Starter Active Member

Sep 27, 2008
239
0
that would work where i must convert from digital to analog the 15 op amps is for analog to digital coversion... Or am i wrong?

4. ### RiJoRI Well-Known Member

Aug 15, 2007
536
29
Sorry, I misunderstood you.

THe 15 op-amps would work, and would be used if you need extremely fast conversions.

Another method is to feed the output from another D/A circuit into a comparator, and feed the analog signal into the comparator also.

A third method is to use an R/C circuit and a clock.

I'll need to look this up once I get home....

--Rich

Apr 5, 2008
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6. ### hgmjr Retired Moderator

Jan 28, 2005
9,029
219
Rather than use opamps in your A-to-D, I think you would be better off if you used voltage comparators instead.

hgmjr

7. ### Papabravo Expert

Feb 24, 2006
12,219
2,699
The big challenge is getting the digital logic between the ADC and the DAC working correctly. An R-2R ladder and a buffer should work for the DAC. You can make a flash converter from precision resistors and comparators. The digital logic in between needs to sample the digital word and transfer it to the output register.

8. ### gusmas Thread Starter Active Member

Sep 27, 2008
239
0
ok ill try that, so where do i start? How do i get the resistor values? I must use the op amp lm741 ic, my vref gona be 16v and i want a voltage drop of 1v per resistor... Is that possible?

Feb 24, 2006
12,219
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