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ADS1248 Digital filter characteristic

Takumi Suzuki1
Mastermind 8420 points
Other Parts Discussed in Thread: ADS1248

Hi,

Let me ask you a basic question about the digital filter as below.

Q1) The digital filter which is implemented on ADS1248 (data sheet 30page, "9.3.5 Digital filter") is for "down-sampling" filter, correct?

Down-sampling means that, for example 2KSPS, 512KHz data is converted 2KHz sampling data. (delt-sigma ADC sampling is 512KHz.) 

My understanding "The digital filter works as down-samploing", is this correct?

Q2) If Q1 is yes,

I think that the characteristic of the filter is not effective to eliminate "aliasing signal".

In case of 2KSPS, the cut-off frequency is 1465Hz and its characteristic is not very sharp. (datasheet 32 page, fig 67)

I think that to eliminate "aliasing", the cut-off must be smaller than 1KHz at least.

ADS1248 data includes aliasing noise if ther are noise. But I also think it must not be that. 

I do not understand well about this....

Could you teach me what I misunderstood and how to understand?

Thank you for your support.

Best Regards, 

  • 0
    TI__Guru* 92655 points
    Takumi-san,


    1. You are correct. The digital filter is "down-sampling" or decimating the output of the modulator. So the input is sampled at 512kHz, but the output data rate is 2kSPS. That means that 256 samples are used to make one output data/

    2. If you want to avoid aliasing, then you need to be concerned with the sampling rate (of 512kHz). With this sampling rate, the Nyquist frequency would be 256kHz. Anything larger than this becomes aliased to a different frequency.

    The cutoff frequency must be smaller than one half the sample rate or 256kHz, and not the 1kHz which is the data rate.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Mastermind 8420 points

    Hi Joseph-San,

    Thank you for your reply.

    I am a little confusing.

    Sampling rate is tha same meaning as "data rate", right?

  • 0 in reply to Takumi Suzuki1
    TI__Guru* 92655 points
    Takumi-san,


    The data rate and the sampling rate are different.

    In the delta-sigma (or oversampling converter), the modulator samples the input at a much faster rate. Then, many modulator samples are used to get one piece of data. In this case, the modulator samples the input at 512kHz, then the output data is at 2kHz. That means 256 samples are used to make one output data.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Mastermind 8420 points
    Joseph-san,

    Thank you for your answer!