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ADS1298: ADS1298 resolution at 8KSPS issue

Richard Legault
Prodigy 10 points
Part Number: ADS1298

Tool/software:

I understand that fast than 8Ksps the ADC has 19 and 16 bit resolution.
But at 8Ksps we are not seeing 24bit resolution. With the REF voltage at 2.5 and 0 we would expect to see 0.298uV resolution but we are seeing 2.98uV resolution, exactly 10x the expected value and I mean exactly when viewing the raw adc values as decimal the last digit for all readings is the same. When we reduce the sampling rate to 4Ksps we are now seeing the full 24bit resolution.

Is this expected but not documented behaviour.

Thanks for your response.

  • 0
    TI__Guru 66186 points

    Hi Richard,

    Thank you for your post.

    I believe the least-significant bits are just fixed in those higher data rate configurations. You will still receive a 24-bit data word for each channel's ADC data. The conversion from codes to volts would be the same, where the LSB (bit 0) carries a weight of 298nV/code. Your effective LSB size will just appear larger as the lower bits will not toggle.

    Let me know if this makes sense or if I'm misunderstanding the problem.

    Regards,

    Ryan

  • 0 in reply to Ryan Andrews
    Prodigy 10 points

    That is what we are expecting and are seeing at 4KSPS but at 8KSPS we are seeing the 4 lower bits only changing in quantizations of 2980nV. ie. the lowest 4 bits only change in increments of 0x0A (ie decimal 10). Is this an artifact of the Filter at 8Ksps.

    Here is an image of the CSV file of the 8 raw ADC readings notice how the LSB decimal digit are all the same.

  • 0 in reply to Richard Legault
    TI__Guru 66186 points

    Hi Richard,

    Yes, this behavior is expected at the higher data rates. The conversion from codes to volts will remain the same, but the lower bits will not all toggle. There is no information to be gained in those lower bits since they are dominated by the increased noise at higher data rates. The digital filter is limiting the available output codes at those data rates.

    Regards,

    Ryan