AD9680-1000 (from AD-FMCDAQ2-EBZ board) - 4 DDC, Complex Input, Complex Output, decimate by 4


I'm trying to configure the ADC9680-1000, for using 4 DDCs. The idea is to split the input band (from -fs/2 to fs/2 in 4, 1 per DDC channel: from -500 to -250, from -250 to 0, from 0 to 250 and finally from 250 to 500) so I will have an effective output sampling frequency of 250 Msps for internal processing in the FPGA (required). 

As far as I understand, I should set the Fc of each DDC (NCO_FTW) and configure the ADC as follows: 

  1. DDC0:
    1. NCO_FTW: 166.67 MHz (the word to load in the registers: 0x314 and 0x315, will be: 0x2AA). POW goes to zero.
    2. 0x310: Complex mixer, 6 dB Gain, variable IF mode, decimate by 4. Which means I'm loading in the register: 0xC0
    3. 0x311: i'm loading 0x4 -> I Input = Channel A, and Q Input = Channel B.
  2. DDC1:
    1. NCO_FTW: 333.33 MHz (the word to load in 0x334 and 0x335 will be: 0x555
    2. 0x330 and 0x331, are loaded as 0x310 and 0x311
  3. DDC2: 
    1. NCO_FTW: -166.67 MHz. So in 0x354 and 0x355: 0xD56 
    2. 0x350 and 0x351, are loaded as 0x310 and 0x311.
  4. DDC4: 
    1. NCO_FTW: -333.33 MHz. So in 0x374 and 0x375: 0xAAB
    2. 0x370 and 0x371, are loaded as 0x310 and 0x311.
  5. The general configurations are:
    1. 0x200: 4 DDC, so I loaded: 0x3
    2. 0x201: 0x2 -> Chip Decimation ratio = 4

With this configuration I spect to receive in the JESD rx_data word (128 bits), from LSB to MSB: [I_DDC0, Q_DDC0, I_DDC3, Q_DDC3, I_DDC1, Q_DDC1, I_DDC2, Q_DDC2] (I checked this distribution using the tests modes).

The JESD links are ok. 

This image corresponds to an input signal of 20 MHz (Just in the channel A of the board, so I spect to read it in the I channels, of DDC0 and DDC2, as a 166,67 MHz - 20 MHz = 146,67 MHz signal), but what I'm seeing is not what I spected:

This second image corresponds to same configuration, but an input signal of 55 MHz in channel A (again, I spect to see it in DDC0 and DDC2):

Any idea of where I would be making a mistake in the configuration?. Any help will be appreciated.

Thanks in advance!



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