• State Resolved
  • Locked Locked
  • Replies 3 replies
  • Subscribers 46 subscribers
  • Views 121 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

ADS8588S: Can a “SAR + digital filter” behavior similar to a delat-sigma？

Woney Gu
Prodigy 130 points
Part Number: ADS8588S

Tool/software:

I am learning the basic operations of SAR and delta-sigma ADC. It is clear that delta-sigma has better noise performance due to its oversampling and internal digital filter. I wonder that if I add an external digital filter to a 16bit SAR, and set the sampling frequency the same as a 16bit delta-sigma ADC, what will be the result of the comparison between two systems' performance? And what is the difference between two systems. Thanks!

  • +1
    TI__Guru**** 174326 points

    Hi Woney,

    SAR converters take a single sample and convert the captured voltage.  Essentially like a camera - take a snap-shot of an instant in time.  Delta-sigma converters start off with a modulator that is basically a 1-bit ADC.  The DS converter is integrating (through the digital filter) these one-bit samples and provide a higher resolution result based on the filter order and oversampling ratio.

    With a SAR, you can take multiple conversion results and theoretically get an additional bit of accuracy by averaging four samples, but it would not be the same thing as using a true delta-sigma converter.

  • 0 in reply to Tom Hendrick
    TI__Prodigy 130 points

    Thanks for your professional answer. Can I understand it in this way?

    For example, an analog signal y=2.1, FSR=4, 2bit, and the sample time is t0=0, t1=1, t2=2, t3=3.

    SAR converters will sample and convert the analog input at t0, t1, t2, t3, and convert 2.1, 2.2, 2.0, 1.9(it may be different from the original input) into n bit digital data for example 10 10 10 01. If a digital filter is added after SAR output, it will average 10 10 10 01 and output only one digital result to represent the analog input from t0 to t3? And this result will be closer to the original input 2.1.

    But when it comes to delta-sigma, if the ref=2, OSR=4, the delta-sigma modulator will work like below:

    time         analog input         ref        modulator output         error

    t0=0         y0=2.1                  2           (2.1>2) 1                     0.1

    t1=1         y1=2.2                  2           (2.2-0.1>2)  1              0.1

    t2=2         y2=2.0                  2           (2.0-0.1<2)  0              -0.1

    t3=3         y3=1.9                  2           (1.9+0.1=2) 1               0

    then, we got a 1 1 0 1 bit stream whose average value is 1.5. The digital filter will average the bit stream, and output a digital result 01 to represent 1(because of resolution). The larger the OSR is, the more accurate the average value of bit stream will be.

  • +1 in reply to Woney Gu
    TI__Guru**** 174326 points

    More or less you have the basic fundamentals.  If t0, t1, t2... represent the data rate of the DS converter, then OSR = 4 means there were four conversions (at least, depends on filter order) that took place to provide t0 output from the DS ADC.