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ADS54J66: ADS54J66: interleaved sample points dismatch

user6050741
Prodigy 110 points
Part Number: ADS54J66
Other Parts Discussed in Thread: ADS54J60

Hi

    The ADS54J66 is  in DDC mode 8  with LMFS = 4421 . The devclk is 500MSPS while the SYSREF is 7.8125MHz.

    With your help, I have received the right Incremental code in the TEST PATTERN.

    But there are other problems about the waveform. 

    I use an amplifier to drive the input signal to ADC through DC-coupling. 

   The received waveform should be a line with small noise on it, but I obtaind a sawtooth wave with about 140LSB amplitude and the histogram statistics of 8192 sampling points is shown below.

   Internally the ADS54J66 have 2 ADCs (@ 250Msps) interleaved to 500Msps. So I wonder  if I need to configure some registers to match the two 250MSPS ADCs. 

  • 0
    TI__Guru* 91947 points

    User,

    This part is not designed for DC coupling, and it appears this might be what you are trying to do. The analog inputs cannot be biased externally as there is no option for this. The common mode is at 2V for these inputs. If you are correctly biasing the input, try disabling the DC offset correction function by setting address 0x68 in page 0x6100 to 0x06.

    Regards,

    Jim 

  • 0 in reply to jim s
    Prodigy 110 points

    Hi Jim

         When I use AC-coupling, the sawtooth wave does disapper. But we still want use the  DC-coupling.

         Does the analog input of the ADS54J66 support DC-Coupling if  the differential amplifier has a  common-mode voltage at 2.0V?  You mentioned the ADS54J60 can support DC-Coupling in the below thread.  

    https://e2e.ti.com/support/data-converters/f/73/t/660047?tisearch=e2e-sitesearch&keymatch=ads54j66%252525252525252520DC%252525252525252520offset

         Besides, I have a question about the datasheet. In chapter 6.5, it's said the input common-mode voltage for analog inputs is 2.0V. While  in chapter 7.3.1, it's said the common-mode voltage of the signal inputs is internally biased to 1.9 V using 600-Ω resistors which allows for ac coupling of the input drive network. In fact, I test the ADC input pins with a oscilloscope, which turn out 2.0V. Is the chapter 7.3.1  incorrect？

           Best regards.

  • 0 in reply to user6050741
    Prodigy 110 points

    Hi Jim 

         In  DC-coupling,  I  adjusted  the the common-mode voltage of differential amplifier, and test the differential analog input pins.  Without any input, the P is 2.07V wihle the N is 2.0V. However, the sawtooth wave  always exists whether the register  610068 is configured or not. The mean vaule of the data is right when  610068 is configured. 

       I'm very confused. I'm looking forward to your reply.

       Best regards.

  • 0 in reply to user6050741
    TI__Guru* 91947 points

    User,

    Please provide more info or a diagram of the input signal. This description "The received waveform should be a line with small noise on it" does not tell me much. The VCM should be 2V. There is an additional supply that gets added to the 1.9V mentioned in the other part of the data sheet.

    Does your setup work with a sine wave? Please check this.

    Regards,

    Jim

  • 0 in reply to jim s
    Prodigy 110 points

    Hi Jim

         We gave the following several types of inputs and confirmed the AD-conversion data. 

         ADS54J66_test.pptx

         best regards

  • 0 in reply to user6050741
    TI__Guru* 91947 points

    User,

    I still do not understand your end application. Is the real input going to be a very low level sine wave? Low level noise?, ect…. Not sure by all of the data you sent. Again, this part is not designed for DC coupling so I do not know if there is anything else I can provide you with.

    Regards,

    Jim