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Output frequency of DAC

Can anybody tell me how to decide the output frequency of the DAC.

I am going to use AD9873 DAC and want 70 MHz output signal from it, For this what should be my sampling rate, clock frequency etc.

If someone can share the calculation,it will be more helpful.

Mukund Jha

  • Hi Mukund -

    The AD9873 transmit path is a direct conversion transmitter that has a signal bandwidth of DC to 65Mhz. I don't think you can use it to generated 70Mhz. You might look at AD9963 for your application.

    The frequency domain DAC output section of the attached poster shows illustrates the considerations you need to take into account in selecting a DAC sampling frequency.

    Thanks,

    Larry

    ADIDataConversionPoster.pdf
  • I am sorry,

    IC is AD9753 DAC.

    I want to get 70 MHz analog output from this DAC. Please tell what should be sampling frequency, clock frequency to get this output.

    Below is some spec of AC linearity part on page 3

    fDATA = 65 MSPS; fOUT = 1.1 MHz  SFDR= 77 dBc

    fDATA = 65 MSPS; fOUT = 5.1 MHz   SFDR = 77 dBc

    fDATA = 65 MSPS; fOUT = 10.1 MHz SFDR= 76 dBc

    How this fout comes for same fDATA. Is fDATA clock frequency of DAC.

  • Hi Mukund -

    Fdata the digital input clock frequency of the AD9753 is 150Mhz maximum. Data is input on Port1 and Port2 in an interleaved fashion.

    The DAC output sampling rate and DAC data input rate are both 2X the digital input clock frequency. The CLK+/CLK- input is 2X

    SFDR should be in terms of Fdac not Fdata. The SFDR charts TPC1, TPC2, and TPC3 are in terms of Fdac. The SFDR numbers in the Dynamic Specifications Table come from these charts. I think use of Fdata is a misprint in the data sheet.

    For a sampling rate you need to be above 2 times your highest frequency. At Fdac = 300MSPS with your signal at 70Mhz, the first Nyquist Zone sampling image will be at 300 - 70 = 230Mhz. 230Mhz is easier to filter out with an LPF following the DAC. As you lower Fdac, filtering the image becomes more difficult.

    Thanks,

    Larry

  • Thanks Larry,

    Just one clarification, how can i vary the output frequency to 30MHz or 70 MHz for this DAC as per my requirement.

    i.e which parameter will determine my output will be 70MHz or 30 MHz.

    Thanks,

    Mukund