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AD9958 - Latency Delay in Linear Sweep

Category: Datasheet/Specs
Product Number: AD9958

I am using the AD9958 to conduct a continuous linear sweep, creating essentially a triangle wave from S0 to E0 and switching right at the start and end frequencies.

My question is what is the delay between the profile pin changing and the sweep changing directions/beginning? The datasheet mentions that the maximum toggle rate for the profile pins is 2 sync clk cycles, but doesn't mention what the internal delay for the sweep to begin is

I am feeding a 100MHz clock to the part and multiplying it up by 5 to get a 500MHz state machine clock and a 125MHz sync clock

Top Replies

    •  Analog Employees 
    May 23, 2022 in reply to dylgan +1 verified

    Hi  ,

    We were able to setup the bench test to check the time delay between switching the pin to the result of the output of the DDS.

    Here’s the output of the setup. Upon checking, the time from the…

  • Hi  ,

    Just to confirm you’re asking about the total delay from the profile pin changing and sweep changing. Just want to confirm for what you meant by sweep changing directions/beginning?

    Are you referring to the Frequency Rising/Falling Delta Tuning Word to DAC output? If yes, upon checking here in the AD9958 data sheet it will take at least 41 Sync Clocks. This data is for the Data Latency Linear Sweep Mode which has a fixed data latency and is referenced to a profile change. So it means it already includes the profile change delay.

    Here’s the table from the AD9958 data sheet for reference:

    Just let me know if you meant something else.

    All the best,

    Jules

  • Yes, I am essentially running the profile pins as a variable clock, where 2 full cycles = modulation rate (direction of sweep = edge of profile pin)

    The exact number I was looking for essentially was how long is the delay between me switching the pin to a result on the output of the DDS, since I am trying to make this sweep as continuous as possible at high frequency sweeps (5-10MHz)

  • Hi  ,

    We were able to setup the bench test to check the time delay between switching the pin to the result of the output of the DDS.

    Here’s the output of the setup. Upon checking, the time from the linear sweep to profile switch, approximately it is 99.2ns.

    Based on the datasheet of AD9958, the minimum time for the data latency linear sweep mode in terms of frequency falling/rising delta – tuning word DAC output is 41 SYSCLKs.

    Computing it in terms of seconds, using the input of 500MHz for the system clock,

    T = 1/500MHz = 2ns

    t = 2ns x 41 = 82 ns , still within the range of the minimum time.

    As for your setup it would be best to mirror the setup we had and perform the same setup for you to get the accurate time using your desired values.

     Here's the setup:

    The bench set up is performed in order to determine the delay between the switching pin profile to the output of the DDS. The channel 1 and 2 of the oscilloscope is utilized in order to display the output waveforms of DDS from DAC0/59 (out) and the pin 2 switching profile. Pin 2 is used as a switching profile to have an output pulse. I’ve attached a picture of the configuration for the linear sweep setup in Channel Output Config window. Frequency Modulation is the enabled modulation output type. The frequency 00 is set to 10MHz while frequency 01 is 30 MHz. The rising and falling step size is both set to 20MHz with rising intervals of 0.008us respectively.  

     

    Note: In order to make changes to the linear sweep setup, you should enable first the linear sweep options in Channel Control.

     

    All the best,

    Jules

  • Thank you for the reply Jules, this is exactly the data I was looking for!