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ADS807E digital outputs are oscillation only the bit which are high are not effected (Any design issue?)

shashi kanth
Prodigy 250 points

Other Parts Discussed in Thread: AFE5807, AFE5807EVM

ADS807E is used in our design, to convert analog input to digital output, expected to work with 50 MHz clock input.

We have referred the design used in DEM-ADS807E Evaluation kit (http://www.ti.com/lit/ug/sbau007/sbau007.pdf)

Problem faced: When an input signal (reference voltage) is given to the input, in the expected digital output most of the bits are oscillating, only the bit which is high is constantly high. If only few LSB was oscillating then it could be the high accuracy reading of the ADC but no matter what bit it is, if it not high than it keeps oscillating from high to low vice versa.

  • 0
    TI__Prodigy 230 points

    I need more information. I assume that you have AFE5807 on the AFE5807EVM.

    Are you using TSW1250 EVM to work with AFE5807E EVM? If the answer is yes then please make sure the bit# option with TSW1250 matches  AFE5807 device. The AFE5807 default to 12-bits. TSW1250 default to 14-bits for the current release. You need to change it back to 12-bits.

    Hi,

    I would like to see a screenshot of what you are seeing, please.  Did you modify the EVM to *not* have the signal AC coupled?  In both the transformer coupled input and the amplifier input options the signal is AC coupled.  For the transformer, the transformer itself won't pass a reference voltage.  With the amplifier, there are series AC coupling caps in the path from the amp to the ADC and this also will not pass a reference voltage.  The data converter may simply be seeing no input, and you may be seeing what we call Idle-channel noise.  With about 0V differential input, it is common to see a few lsb's toggling due to noise.  And if the DC offset of the data converter is less than the Idle-channel noise then the output codes would be flipping about mid-scale which would make *all* the digital outputs be toggling.  In this case, you should see most of the msb's be the same state on a given sample as the output code goes from <0111 1111 1111 to >1000 0000 0000.

    Regards,

    Richard P.

  • 0 in reply to Allen Chan
    Prodigy 250 points

    Dear Richard,

    Thank you,

    I have attached the schematics for your reference,

    What ever the input given, the bits are oscillating, Only in out of range condition depending on the input all bits are set to 1 or 0.

    Please advise.

     

    Best Regards,

    Shashi.2318.TI_SCHEMATIC.pdf

     

     

  • 0 in reply to shashi kanth
    Prodigy 250 points

    Any Updates Please.

  • 0 in reply to shashi kanth
    TI__Mastermind 29220 points

    Hi,

    When you said you referred to the DEM-ADS807e evaluation kit, I thought you meant you were evaluating that board, not that you had your own board that was done with reference to that EVM.  Thanks for the clarification and the schematic for your board.

    For your given input voltage, what do you see at the analog inputs in+ and in- with an oscilloscope?  If your input voltage is a DC signal, do you see a DC signal at in+ and in- such that in+ is as much above the desired common mode voltage of 2.5V as the DC signal at in- is below that common mode voltage?  (from the original posting, i think you are using a DC input voltage to the amp circuits as you called it a reference voltage.)  May I see a screen shot of the input signals at in+ and in-, since the output codes seem to not be interesting enough to capture if they just toggle between all 1's and all 0's.  Do the output codes alternate like this on every sample clock, or is the output of all 1's and all 0's more random?

    Regards,

    Richard P.

  • 0 in reply to Richard Prentice
    Prodigy 250 points

    8372.TI.docx

     

    Dear Richard,

    Thanks for your replay,

    Please find the attached document for detailed report.

     

    Best Regards,

    Shashi.