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ADS8689: Incorrect conversion data

Zeming Kong
Expert 3515 points
Part Number: ADS8689

Hi team,

My customer is evaluating ADS8689, he found the conversion data of 1st cycle is incorrect, but the data of next cycles are correct, what could be the problem?

Best,
Zeming

  • 0
    TI__Guru 66466 points

    Hello Zeming,

    Thank you for your post.

    It's possible that either the power supplies have not fully ramped or the input signal has not settled after startup. Can the customer experiment with adding some delay before they begin ADC conversions?

    Regards,

    Ryan

  • 0 in reply to Ryan Andrews
    TI__Expert 3515 points

    Hi Ryan,

    Thanks a lot for your reply.

    Actually, the customer set ADDR 0x14 value to 0x0B (unipolar 1.25x Vref) and waited for long enough time to start the data conversion, but the 1st data did not show the new settings, the next data cycles were correct. Thus, we think it is not a problem caused by startup.

    Do you have any other idea?

    Best,
    Zeming

  • +1 in reply to Zeming Kong
    TI__Guru 66466 points

    Hello Zemig,

    I believe it does make sense to discard the first ADC sample. Notice in Figure 6-1 that the conversion period occurs first, followed the acquisition of the next sample voltage. During the first conversion period, the sampled voltage is from the "previous acquisition," meaning whatever voltage was present on the internal sampling capacitors during startup is the value being converted. Once you convert this value, the next acquisition period will sample the proper input signal.

    Regards,

    Ryan