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ADS1256 Normal Mode Rejection

TonyMCC
Intellectual 780 points

Other Parts Discussed in Thread: ADS1256

I have a couple of question with regard to the ADS1256 digital filter response.

1. There does not seem to be a specification for the Normal Mode Rejection at 50/60Hz. How would I determine the Normal Mode Rejection specification for each data rate and PGA gain, with buffer ON ?

2. I modeled the digital filter frequency response in MathCad using the formula given on pg 18...can I assume that the filter's "notch" will theoretically extend to -infinity ?  If this is the case, why does the y-axis of the 2.5sps frequency response graph in figure 15 only extend to -60db ?

  • 0
    TI__Mastermind 44790 points

    HI TonyMCC,

    1. The normal mode rejection is the digital filter response at 50 or 60 Hz. Typically, a frequency range of +/-2% around the normal mode frequency (49 to 51 Hz or 58.8 to 61.2 Hz) is measured and the minimum filter attenuation is quoted as the normal mode rejection. The 2% frequency range is used because it is not possible to sample at exactly 50 or 60 SPS (The crystal oscillator frequency stability may only be 50ppm and parasitic capacitance may change the crystal's frequency), AND ac power lines are not perfectly stable but drift slightly in frequency too!

    2. Yes, this notch does go to negative infinity; however, it would be very impractical to graph! At a minimum y-axis value of -60dB, you get a good idea of the filter's transfer function. Note, this notch is so narrow that we really don't benefit from the infinite attenuation due to the effects mentioned above.

    Best Regards,
    Chris

  • 0 in reply to Christopher Hall
    Intellectual 780 points

    Thanks Chris, your test method description helps to clarify things for me.

    I went back and reviewed the datasheet but didn't see the Normal Mode Rejection specification (unless I missed it). Can you provide the NMR specification, or method to derive for each data rate of the ADC.

  • 0 in reply to TonyMCC
    TI__Mastermind 44790 points

    Hi TonyMCC,

    It's found by graphing the digital filter response (see attached): 0160.ADS1256 Digital Filter.xlsx

    Here is the filter attenuation at the frequencies I mentioned above:

    Note: Highlighted are the 50/60 Hz notches. SINC filters have the property that notches occur at multiples of the data rate.

    Best Regards,
    Chris

  • 0 in reply to Christopher Hall
    Intellectual 780 points

    Got it...thanks Chris.