• State TI Thinks Resolved
  • Locked Locked
  • Replies 5 replies
  • Answers 2 answers
  • Subscribers 46 subscribers
  • Views 484 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

DAC902: DAC902

Mohammad Al Abdullah1
Prodigy 30 points
Part Number: DAC902

I'm using DAC902 @ clock rate of 40 MSPS. The DAC is generating a 1 MHz sine wave. when I measure the output of the DAC, I get the following noisy sine wave:

 

I'm not sure what's the reason behind this. I did notice that in the datasheet, they didn't mention a clock rate of 40 MSPS, so I'm not sure if that's the reason behind the problem.

Below is a picture of the schematics:

Is it normal for the output of the DAC to be that noisy?

Can anyone give me any suggestions?

Thanks

  • 0
    TI__Guru* 91947 points

    Mohammad,

    Make sure the load on the DAC output pins does not exceed 25 Ohms each. I am not sure if this is true with your design. See figure 4 of the data sheet for more information about interfacing to an amplifier. If your load is set properly, check the DAC output at the output of U26.1 and not the ADC on your design. If the output looks bad here, verify the setup and hold time of the data going to the DAC. This is usually the issue if you slow down the sample rate and it cleans up the output.

    Your first capture of the output appears as is if the data going to the DAC is out of order.

    Your second plot looks like either a timing issue or noisy clock. Try filtering the clock to see if this helps. Make sure the clock amplitude is within the specs of the data sheet. 

    Regards,

    Jim 

       

  • 0 in reply to jim s
    Prodigy 30 points

    Thank you for your reply,

    In figure 4 of the datasheet, is the meaning that R1 is in parallel with RL and R3+R4 are in parallel with RL? 

    If that is the case, then my loading is wrong. Another thing is, should I expect the output to be super clean or do I need to add a reconstruction filter?

    Is the 40 MSPS ok for this DAC? I didn't see it mentioned in the datasheet and I'm not sure if I'm doing something wrong there.

    Thank you

  • 0 in reply to Mohammad Al Abdullah1
    TI__Guru* 91947 points

    The data sheet mentions RL is in parallel with R1 and the input impedance of the amplifier. I am not an expert with amplifiers so I cannot provide a better answer than that. You may want to post this with the amplifier group.

    The clock rate should not be an issue. There are several measurements in the data sheet with the DAC running at lower speeds. You should not need a reconstruction filter but may need a clock filter. Make sure your clock is clean and the input data is meeting setup and hold time. 

  • 0 in reply to jim s
    Prodigy 30 points

    Thank you very much for taking the time to help me out. I will check out the things you mentioned.