digital to analogue converter

Discussion in 'General Electronics Chat' started by dude521, Mar 24, 2009.

  1. dude521

    Thread Starter Active Member

    Nov 1, 2008
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    0
    Hi guys,

    I'm using a DAC (the AD569JN, http://www.analog.com/static/imported-files/data_sheets/AD569.pdf) which takes a 16 bit number and outputs that as a percentage of its range. So for example I set up the DA to have a range of 0 - 5V (I have 5.2V on pin 3 and 5V on pin 2, 0V on pin 16 and -0.2V on pin 15). So if I want 5V I output 65535 (0xFFFF). That should give me exactly whatever voltage I have on the +Vref input. But instead I'm getting ~4.54V. Infact, all the voltages are scewed. If I output 13107 (0x3333) which is 1/5th of 65535, I should get 1V, but instead I get 0.91V. The forumla I'm using is this:

    outputCode = 65535 * (desiredVoltage / 5);


    I'm using a high precision DVM to measure these voltages, and I need the output to be as exact as possible. Is there some sort of trick to getting the correct output? I've read the datasheet several times and it seems like this should work. All the input voltages are exact, but the output just doesn't work.

    BTW, I have 3 of these DACs and i've tried them all, they all give the same result.


    Thanks.
     
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    This may seem a dumb question - but do you get 0V out with 0x0000?

    If so are all the the output values under target by the same ratio?
     
  3. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
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    Also - exactly how have you set up the (5V) reference input? I realize you've given values at the 4 ref input points, but a circuit diagram of your setup would possibly help in resolving your problem. Show the +/- Vs values, etc.
     
  4. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    What does your meter display for the supply voltages and the reference voltages?
     
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    If you are applying voltage sources to pins 2 and 16, that explains why you are getting the results you listed. You need to apply your reference voltages (0V and +5V) to the inputs of voltage followers, as in Fig. 10. The forcing voltages at pins 3 and 15 will be whatever they have to be to get +5V on pin 2 and 0V on pin 16.
     
  6. dude521

    Thread Starter Active Member

    Nov 1, 2008
    37
    0
    Thanks guys, I found the problem. I as trying to set it up like figure 10 in the datasheet, where you use op-amps to create a reference voltage and a forced voltage. I had a wiring problem that I now got resolved.
     
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