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ADS8598H: Serial Mode Data Inversion

Errol Murray1
Prodigy 20 points
Part Number: ADS8598H


Hi, I’ve designed and constructed a prototype that uses three of the ADS8598H ADCs (for a total of 24 channels) each of which is configured in Serial Mode. The hardware configuration for all three ADCs is identical (they are each instances of the same design). Two of the ADCs work as expected, however the third outputs data that is inverted on the DOUTA & DOUTB lines (I.e a zero value is output as 111111111111111111 instead of 000000000000000000). I have verified all of the settings pins (RANGE, REFSEL etc) with an oscilloscope and all three are consistent. I have also disconnected the output data lines from the MCU that I’m using and the lines are still inverted (so it isn’t a conflict with the MCU). Any ideas or suggestions on what might cause this issue? Could it be a timing issue, power sequencing issue or something obscure? I’m quite stumped at this point. Cheers, Errol
  • 0
    TI__Mastermind 46035 points

    Hi Errol,

    Just confirm, are you sharing any signals/pins from your controller for these three ADCs? or using individual SPI interface for each ADC? Since you did the test by using dedicated lines for this ADC, the third ADC should work well if same software code from other two ADCs is used for this third ADC. I would say this may be a connection or hardware issue. I would suggest:

    • Keep dedicated signals for this third ADC, no any connections to other devices on the circuit board.
    • Check the voltage on REFIO pin 42, also the voltage on REFCAPA and REFCAPB.
    • Apply a DC signal to the analog input, or leave it float which will be a voltage input higher than 0V.
    • Provide your timing plot for SCLK,/CS,DOUT and BUSY and CONVST captured by an oscilloscope
    • Also, I need your schematic which will be helpful to address the issue. You can share it with me by friendship if you would not upload it here.

    Thanks&regards,

    Dale

  • 0 in reply to Dale Li
    Prodigy 20 points

    Hi Dale,

    I appreciate you taking the time to write a response!

    I checked all of the items that you've suggested above but still couldn't find any issue.

    However after playing around for a while I realised that the ADC channels that appeared negative were all connected to a different sensor AFE than the other ADCs. Whilst all of the ADCs should have shown a value of zero, all of those amplifier stages were introducing a negative offset that pushed all the signals slightly negative (~ -3mV) which because of the two's complement nature of the data made those channels appear inverted.

    Additionally there was a small error in my algorithm for decoding the data for that ADC which meant the decoded data looked wrong and the oscilloscope data looked wrong but it wasnt the ADC.

    Turns out the ADC works perfectly.

    Appreciate the response.

    Cheers,

    Errol