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ADS127L21: 50/60Hz filter

Theo Keinarth
Prodigy 160 points
Part Number: ADS127L21

Hi TI-Support Team

We are looking for a solution on how to filter 50/60Hz noise from a SIL2 anlog 4-20mA loop signal. But to be honest im not sure if this is a bit to much for just filtering 50/60Hz from an anlog signal. Our required AC performance is only 20Hz. So my questions are: What would be the best filter ind the ADS127L21 for that application (SINC1+ SINC3, FIR1 etc.)? Do I need a external oscillator or can I use the integrated clock? If needed how do I calculate the FIR/IIR coefficients? If i activate the SPI CRC in the CONFIG3 register do I need to send a CRC with that command already or only with all subsequent massages?

I do have a EVAL board already.

Thanks in advance.

Regards Theo

  • +1
    TI__Mastermind 37570 points

    Hello Theo,

    The SINC filters will have too much attenuation (>10dB) at 20Hz if set to filter 50/60Hz.  The best approach is likely to use a custom FIR filter with a corner frequency set a little higher than 20Hz.  Attached is custom filter designed with MATLAB that can be loaded into the ADS127L21EVM using the GUI.  The filter has a flat response out to 30Hz, and 80dB of attenuation from 45Hz and higher.

    ADS127L21_FIR_25Hz_250sps.fcf

    Below is an image of the ADS127L21EVM-PDK-GUI for the proper settings and an approximate frequency response with the attached filter.

    You can use the internal oscillator, but the sample rate, and corner frequency of the filter will vary from part to part by +/-0.8%.

    If enabling SPI CRC, you only need to include the CRC byte with all subsequent commands.

    Regards,
    Keith Nicholas
    Precision ADC Applications

  • 0 in reply to Keith Nicholas
    Prodigy 160 points

    Hi Keith

    The Respons looks exactly what i was looking for. Thank you very much for helping me with that. The variation of +/-0.8%  is fine for our purpose.

    Regards Theo