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ADS4449: dataBit fluctuation

Naiwen Chang
Prodigy 140 points
Part Number: ADS4449

Hi,

I have a design problem

Parts:ADS4449IZCR. 

After AD sampling, it enters FPGA and captures data

When no input signal or input DC = 0V, there are still some dataBit such as Bit_4 and Bit_5 fluctuate

picture below is data_D

picture below  is data_C                                                         

DC-couple scenario

Under what circumstances may cause dataBit fluctuation according to your experience ?

  • 0
    TI__Guru* 91947 points

    Naiwen,

    Make sure to reset the ADC, program all registers bit with "1" for the addresses that mention in the data sheet to always write "1", enable the digital functions by writing 0x08 to address 0x42, enable Offset Correction EN1 by writing 0x20 to address 0x3D, and enable Offset Correction EN2 by writing 0x08 to address 0xCF.

    Regards,

    Jim

  • 0 in reply to jim s
    Prodigy 140 points

    thanks a lot~

    but bit still fluctuates. besides, after setting up digital functions / offset correction1 / offset correction 2 the max value of FPGA decode turns from 8192 into 8063; the minimum value turns from -8192 into -8063. Is that normal for ADS4449 ?

    thanks for advice~

  • 0 in reply to Naiwen Chang
    TI__Mastermind 23210 points

    Hi Naiwen,

    Since you are using the ADC with a DC coupled frontend. Please check to ensure the VCM on each analog input pin is 1.15VDC.

    Regards,

    Rob

  • +1 in reply to Rob Reeder
    TI__Guru* 91947 points

    Naiwen,

    Your system is probably not properly terminated when you have no input, thus causing some of the upper bits to toggle. You should only see the lower 4 bits toggling at most. With no input on the TI EVM, the mean count was about 8191 (unsigned binary) with the minimum at 8185 and the max at 8199 when capturing 65,536 samples (see attached). 

    With the digital functions enabled, the device will not operate properly with saturated data due to the internal digital logic. The logic must have valid data to operate properly. This is not an issue if operating with the input below saturation. Most users typically operate with an input that is -1dBFS, which the plots are showing in the data sheet.

    Regards,

    Jim 

    Input open digital functions enabled.pptx

  • 0 in reply to jim s
    Prodigy 140 points

    Jim, 

    I will check my system.

    Thanks