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ADS1257: How do you think of "Single-Cycle Settled Conversions"?

Takumi Suzuki1
Mastermind 8490 points
Part Number: ADS1257
Other Parts Discussed in Thread: ADS124S08

I would like to ask you about very basic question, "Single-Cycle Settled Conversions"?

Basically, a delta-sigma A/D is used as "continuous" conversion due to internal digital filter.

Why did you develop "Single-Cycle" conversion of delta-sigma?

I suppose you intend to let us use delta-sigma as like "SAR" with high resolution and with MUX.

Is my guess correct?

And recently, as high resolution SAR seems to be getting to be RTMed,

I think it might be better to use such kind of SAR in case of multi-input signals than delta-sigma. 

I appreciate your support.

Best Regards,

  • 0
    TI__Guru**** 174226 points
    Hi Takumi,

    Your guess is not really correct. 'Single-cycle' delta-sigma converters have been around for a while, and basically what they do is only provide you with a fully settled conversion result. The intermediate filter results are not output to the user. As related to a SAR converter, the delta-sigma takes many samples and basically averages them together. The SAR takes a single sample and provides a single output result.
  • 0 in reply to Tom Hendrick
    Mastermind 8490 points

    Hi Tom-san,

    Thank you so much for your reply!

    But let me ask you additional questions as below.

    1) What is the merit to use "single -cycle settled conversions"  to comparing with SAR?

    2) What kind of application should we use this single-cycle settled conversions?

    3) Is it possible to use "Single-cycle settled conversions" as like SAR?

        For example, in case 1Hz sampling, 

              1st data is got by single-cycle with 2SPS(0.5Sec) -----after 0.5 Sec-----> 2nd data is got by single-cycle with 2SPS ----after 0.5 Sec -----> 3rd data is got 

       I think, in case of single-cycle wiht 2SPS, as the data is outputted for about 0.5Sec, so we can do above sequence.

      That is to say, we can get the data every 1Sec by delta-sigma ADC.

    I'm very sorry to ask you for my too basic questions.

    Thank you so much in advance.

    Best Regards,

  • 0 in reply to Takumi Suzuki1
    TI__Mastermind 44620 points

    Hi Takumi-san,

    I can help with these questions in addition to your previous filtering question (e2e.ti.com/.../2722143

    1) Most SAR ADCs do not have a digital filter so filter settling is not an issue. Delta-sigma ADC's operate off of a different principle (oversampling, noise shaping, and filtering) to achieve higher resolution; however, latency and data rates rate are slower than SAR ADCs.

    2) Any application that benefits from lower latency but requires higher resolution than a SAR ADC. A good example is a MUX'ed delta-sigma ADC that switches channels. For SINC2, SINC3, and SINC4 digital filters the settling time is 2, 3, and 4 conversion periods respectively; therefore when MUX'ing between channels, the effective throughout per channel is slight slower then 1/2, 1/3, and 1/4 the output data rate (respectively) divided by the number of channels.

    3) With the digital filter the first conversion will always be slightly delayed, but on-going conversions will have a conversion period of (1 / data rate). FYI: The ADS124S08 digital filter is our fastest settling digital filter, so the settling time vs data rate will be the closest with this device, when compared to other delta-sigma ADCs with similar data rates.

    I hope I have I helped to clarify your understanding of "single-cycle settling"; however, please let us know if you still have additional questions!

    Best regards,
    Chris

  • 0 in reply to Christopher Hall
    Mastermind 8490 points
    Chris-san,
    Sorry for my late response.
    I have studied and I think I understood.
    Thank you so much!
    Best Regards,