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    A reconstruction filter for delta sigma DAC

    Hello all,
    I'm designing a 16bit Audio Delta-Sigma DAC using 180nm technology. The purpose is to learn how to make a delta-sigma DAC at the first try while high performance is not a concern. therefore I chose 44.1KHz as the input sample rate without using oversampling. I have the bitstream now but I can't filter it out. Do I have to use SC filters? what are the options?

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    Re: A reconstruction filter for delta sigma DAC

    Delta-Sigma DAC uses oversampling by design. The oversampling ratio is chosen depending on the quantizer bit width and intended SNR. Due to high sampling rate, a simple first or second order filter is often sufficient for output "reconstruction".



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    Re: A reconstruction filter for delta sigma DAC

    I studied that Interpolation filters are deployed to oversample the input data toin order to shape the noise(High pass the noise), remove some order of Fs replicas, and ease the reconstruction filter design(So we don't have to design a fast slop at fc).
    But there still are people who do not oversample. I'm new at this topic, and I want to keep things simple for now :)



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    Re: A reconstruction filter for delta sigma DAC

    In my view, delta-sigma converters are strictly linked to oversampling, review basic literature like Schreier, Understanding Delta-Sigma Data Converters.

    Apart from commonly understood terminology, what do you want to achieve? If you operate the converter at 44 kHz input rate, how many bits of quantization do you implement? What's the expected signal-to-noise ratio?

    Regarding reconstruction filter, a quantizer operated at input data rate usually needs a high order analog filter.



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    Re: A reconstruction filter for delta sigma DAC

    Quote Originally Posted by Kamran1992 View Post
    I have the bitstream now
    but I can't filter it out.
    Do I have to use SC filters?
    No.
    Can you understand a sampling theorem correctly ?
    Reconstruction filter is for reducing image, that is, alias spectrum.
    SC filters is not helpful for it at all.

    Quote Originally Posted by Kamran1992 View Post
    I studied that Interpolation filters are deployed to oversample the input data
    ..................................
    remove some order of Fs replicas,
    No. Interpolation filter in DAC is digital filter.
    There is no effect to remove some order of Fs replicas at all.

    Quote Originally Posted by Kamran1992 View Post
    and ease the reconstruction filter design(So we don't have to design a fast slop at fc).
    Right.

    Quote Originally Posted by Kamran1992 View Post
    But there still are people who do not oversample.
    Purpose of Oversampling

    (1) Reducing PSD(Powerm Spectrum Density) of Quantization Noise
    (2) Quantization Noise Shaping for achieving high SNQR over interest bandwidth
    (3) Relaxzation for requirement of reconstruction filter

    Delta-Sigma-DAC is (2).

    (3) is not limited to Delta-Sigma-DAC.

    Quote Originally Posted by Kamran1992 View Post
    therefore I chose 44.1KHz as the input sample rate without using oversampling.
    Quote Originally Posted by Kamran1992 View Post
    But there still are people who do not oversample.
    DAC output is zero order hold waveform where high order replica spectrum is reduced accordimg to sinc() characteristic.

    However steep cutoff characteristic(at 22.05kHz) LPF is required.
    Last edited by pancho_hideboo; 7th September 2019 at 08:10.



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    Re: A reconstruction filter for delta sigma DAC

    Hi,

    But there still are people who do not oversample.
    With audio? For somehow - at least mediocre sound - I doubt it.
    Even my first CD player from 1985 uses oversampling technique.

    And as already said: delta sigma is always oversampled.

    I'm designing industrial measurement equippment - non audio.
    Even in this field proper analog filtering, nyquist, oversampling ... are very useful to take care of...

    Klaus
    Please don´t contact me via PM, because there is no time to respond to them. No friend requests. Thank you.



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