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ADS1298 Noise Shaping

David Ryan
Intellectual 295 points

I don't understand the "Modulator Noise Spectrum" figure in the ADS1298 datasheet (Fig 29).

It says it's showing Normalized Frequency, but what was the frequency normalized by?
Usually "normalized" frequencies are unitless, but it says the units are Hz and the x-axis shows data up to 700Hz.
The figure title says "Modulator Noise Spectrum Up To 0.5 × fMOD," but fMOD is either 256kHz or 512kHz depending on the operating mode (Low Power/High Resolution).

Can someone make sense of this?

Thanks!

  • 0
    TI__Mastermind 43915 points

    Hi Dpryan,

    Figure 29 in the data sheet is more or less an example shown out to 1k when using a 2k modulator clock. We wanted to show the noise shaping out to fmod = fclk/2 and not necessarily the behavior of the converter. We try to explain that with the sentence As in the case of any delta sigma modulator, the noise of the ADS129x is shaped until fmod/2. Generally the details of the noise shaping is not always given out in delta sigmas where you do not have access to the raw modulator data. I agree that it would have been better off if we left the x-axis of the graph in terms of a ratio of fmod. Give this guy a read to understand more details on modulator noise shaping.

    Regards,

    Tony Calabria

    CTdeltasigmaModulators.pdf