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ADC12J1600: Fundamental tone attenuation

Part Number: ADC12J1600


I set up ADC12J1600 using HSDC pro as per this thread:

  • Initially, I tried a 2MHz single ended tone with 600Mvpp swing and got severe attenuation in the FFT plot (Fundamental = -70dBFs).
  • Next, I bypassed the onboard BalUn and provided differential inputs with a 450mVpp swing on each at a 1.225V common mode. (ac coupling caps C1 and C6 intact). The output still remained the same.
  • Finally, I removed the ac-coupling capacitors(C1 and C6) and shorted the traces to provide a dc coupled inputs of 300mVpp swing at 1.225V common mode. The FFT still shows -61dBFs for the fundamental

  • An observation was that, when the swing is 100mVpp and the VCMi = 150mV, at 7MHz, the FFT shows ~-49dBFs for the fundamental. The FFT spectrum is shown here

  • The codes show 16340(saturating input).

Is there a way to differentiate ac coupled vs dc-coupled inputs from the ADC point of view? Is there something I am doing wrong? Please help. The max frequency content of my signal will be 5-7MHz.

  • Hi Mahesh
    Since you are using Decimate-by-4 mode the signal input to the ADC and converted is then processed by the complex mixer and DDC filter.
    The default NCO frequency input to the mixer will be 3/4 of the ADC clock rate. For 1600 MHz clock rate the NCO will be at 1200 MHz. This will result in frequencies at 1200 MHz being shifted down within the pass-band of the decimation filter.
    If you want to process signals that are already near DC then you should change the NCO frequency setting to be 0. In the ADC12J1600EVM GUI navigate to the NCO Configuration tab and change the Preset 0 Frequency to 0. This will change the value in Register 0x213h from C0h to 00h.
    Once that is done the signals you are applying will show up in the data output by the device.
    Best regards,
    Jim B
  • Hi Jim,
    Thank you.
    This works now. So, is our assumption the data rate and the framing is still as before? What did this NCO setting change? If NCO to DDC is 0, doesn't it mean no decimation?
  • Hi Mahesh
    Please refer to Figure 63. DDC Details Block Diagram in the ADC12J1600 datasheet.
    The NCO setting changes the frequency of the oscillator (digital Numerically Controlled Oscillator) which feeds into the complex mixer section. Changing the NCO frequency shifts the ADC output spectrum that is input to the decimation filter. Some portion of the spectrum centered around DC will be passed and output in the data. Frequencies outside the DDC bandwidth are eliminated.
    The decimation factor is determined separately by the Decimation register setting. This adjusts the output sample rate and resulting output bandwidth, but does not select what portion of the ADC Nyquist zone is output.
    Best regards,
    Jim B