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ADS1605: A question about Δ-Σ ADC

user5902932
Prodigy 20 points
Part Number: ADS1605



I found that the normal Δ-Σ ADC has a sampling frequency of up to 5M and a bandwidth (BW) of 2.45M (ADS1605). For isolated Δ-Σ ADCs, the sampling frequency can be as high as 20MSPS, but the bandwidth is only 800K. (AMC1304x-Q1) Why is the sampling frequency of the isolated ADC so significant? But the bandwidth is reduced? Why are isolated ADCs based on delta-sigma modulators, and other structures (such as SAR or Pipeline) not?

  • 0
    TI__Mastermind 47340 points

    Hello,

    Welcome to the TI E2E community.

    ADS1605 is a delta-sigma ADC that integrates a digital filter.  It samples the input at 40Msps, and provides an output data rate of 5Msps, supporting input signal bandwidths up to 2.45MHz.

    The AMC1304 is a delta-sigma modulator and does not include a digital filter.  It samples the input at 20Msps, and after a digital filter is applied to the modulator output, the output data rate will be much lower.  In order to get good performance on the AMC1304, the digital filter will limit the bandwidth to less than 1MHz.

    The delta-sigma modulators only require two isolation channels to transfer data, which is why they are popular when isolation is needed.  A SAR or Pipeline would require many more isolation channels (at least 4 channels for a SAR).

    Regards,
    Keith Nicholas
    Precision ADC Applications

  • 0 in reply to Keith Nicholas
    Prodigy 20 points

    hello,

    thanks for the answer, but I have still some questions:

    1. How can man understand the "output data rate of 5Msps" and the "input signal bandwidths up to 2.45MHz"?

    is it means, that the cut-off frequency of Digital Filter is 2.45M？  as in the picture:

    is Fs=5M        KFs=40M   and Fa=2.45MHz     right?

    2. why require the delta sigma modulators only two channels? The output of delta sigma is actually a one-bit bitstream. is the another channel of CLOCK?

    Regards

    Yizhen Tian
    MAHLE International GmbH
    (CR)

  • 0 in reply to user5902932
    TI__Mastermind 47340 points

    Hello Yizhen,

    1.  Yes, the digital filter inside the ADS1605 sets the input signal bandwidth to 2.45MHz when running at an output data rate of 5Msps.

    2.  Yes, a delta sigma modulator requires two digital lines.  Serial Out, one-bit bitstream, and a serial CLOCK.

    Regards,
    Keith