Crosstalk about AD9653

Dear Sir/Madam,
         I am designing a NMR Signal Acquisition Board. What it does is receiving the analog signal modulated at 63.8MHz, and then converted into digital signal using AD9653. And then the digital signal enters the FPGA for downsampling, downconversion, Cascaded Integrator–Comb (CIC) decimation filtering, etc. After processed by the host computer, the image in the first picture can be got.
         However, the question is the final image has a 180 degree flip shadow.
         The second picture is the input of AD9653 when AD9653 is not working.
         The third picture is the input of AD9653 when AD9653 is working.
         The sampling frequency used is 100Mhz.
         It seems that crosstalk appears between AD9653 and the pre-stage analog circuit.
         So, could you offer me some suggestions for solving this problem?


Best regards
Chen

Parents
  • Hi Chen,

    Thanks for using the AD9653 in your system.

    • You said that "The second picture is the input of AD9653 when AD9653 is not working." When you say "not working", does that mean that the AD9653 is powered down?
    • The AD9653 has switched capacitor inputs. As such there will be a lot of switching artifacts at the ADC inputs. In the time domain these occur between sampling instants, so they do not cause a problem. In the frequency domain I'm not sure how they will appear, but there will certainly be additional tones compared to when the ADC is powered down.
    • You mention that you suspect the signals are coming from the pre-stage analog circuit. Are the frequency components you see in the third picture what are present in the pre-stage?
    • Have you confirmed that the AD9653 outputs are being captured correctly? The AD9653 has output digital test patterns in case that would help.
    • To be sure the AD9653 is in the proper operating state, would you please do a Digital Reset on AD9653 after your system is powered up and in steady state?
      • Write Register 0x08 = 0x03
      • Write Register 0x08 = 0x00

    Thanks,

    Doug

Reply
  • Hi Chen,

    Thanks for using the AD9653 in your system.

    • You said that "The second picture is the input of AD9653 when AD9653 is not working." When you say "not working", does that mean that the AD9653 is powered down?
    • The AD9653 has switched capacitor inputs. As such there will be a lot of switching artifacts at the ADC inputs. In the time domain these occur between sampling instants, so they do not cause a problem. In the frequency domain I'm not sure how they will appear, but there will certainly be additional tones compared to when the ADC is powered down.
    • You mention that you suspect the signals are coming from the pre-stage analog circuit. Are the frequency components you see in the third picture what are present in the pre-stage?
    • Have you confirmed that the AD9653 outputs are being captured correctly? The AD9653 has output digital test patterns in case that would help.
    • To be sure the AD9653 is in the proper operating state, would you please do a Digital Reset on AD9653 after your system is powered up and in steady state?
      • Write Register 0x08 = 0x03
      • Write Register 0x08 = 0x00

    Thanks,

    Doug

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