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ADS129x Sampling Accuracy

Vala
Expert 1110 points
Other Parts Discussed in Thread: ADS1298R

Hi,

I know that the internal clock source of ADS1298R introduces a small error in the sampling rate and the direct solution is to include an external clock source. The drawback is that I would need some space on my PCB for this additional component.

My question is if I can reduce this error by single shot readings when using the internal clock source. An advantage that I would achieve in this case is the non-standard data rates. But the drawbacks would be more aliasing effects and required high sampling comparing to data rate to address the problem of settling time.

Can I get a more accurate sampling with single shot measurement? If so, how much more accurate? What are the error sources in this case? Of course everything is again related to internal clock frequency, but how does it affect on taking one sample? This is while assuming that the START command (not START pin, I don't have IOs available for it), comes from a very accurate interrupt service routing.

-Vala

  • 0
    TI__Mastermind 38745 points
    Hello Vala,

    The ADS1298R is a delta-sigma ADC, which means that any given conversion result is actually the average of many conversions over a period of time. Therefore I'm not sure that it is appropriate to speak of "sampling accuracy" in the way you probably mean. Unlike a SAR ADC, a delta-sigma, even in single-shot mode will collect hundreds or thousands of samples during the conversion cycle.

    However if your clock source had significantly better accuracy than the internal oscillator of the ADS1298R, you could start single shot conversions according to that clock. And even if the ADS1298R conversion clock has error in it, the error will always be approximately the same (assuming the temperature remains relatively constant). Therefore I'd say that yes, you can improve the timing error that way.

    However, you pointed to a major drawback of this method. The quantization noise from delta-sigma ADCs increases significantly at higher data rates. This method requires a higher data rate in order to allow the ADS1298R's digital filter to settle in between each conversion. To retain acceptable noise performance, it may be required that you filter the output data yourself in software, but it depends how susceptible your application is to noise.

    Regards,
    Brian
  • 0 in reply to Brian Pisani
    Expert 1110 points
    Hi Brian,

    Thanks for your answer. Yes you are right I should have said the data rate accuracy.