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8kHz output from ADS1282 can be suppressed?

Other Parts Discussed in Thread: ADS1282, ADCPRO

Hello,

For some reason ads1282 is used on a high sample rate, e.g. 32ksps or 64ksps. This is done by only using SINC filter on ads1282. But 8KHz output is shown in  the conversion results as descriped on page15 of ads1282 datasheet, how can I remove the 8KHz output from ads1282 itself. It seems disabling chopping feature does not work.

Best Regards

Yang

  • Anyone can give some hints about this?
  • Yang,


    Unfortunately, even with the CHOP disabled for the PGA chopping, there may still be an 8kHz spike. The ADC modulator itself also uses chopping and it runs at the same 8kHz clock rate. If you are running the EVM with ADCPro, you still can see it and it was noted in a previous post:

    e2e.ti.com/.../412206


    Joseph Wu
  • Hi Yang,

    Some additional information...

    The ADS1282 is typically operated with the FIR filter, which removes the higher-frequency noise. 

    The SINC filter output bypasses the FIR filter to allow for faster data rates; however, the higher-frequency noise is no longer removed. In most cases when this mode is used, the system will implement a custom digital filter to supplement the ADS1282's SINC filter.

    ...If you only need to remove the 8 kHz niose, you could run the ADS1282 in SINC filter mode at 8 kSPS to place a SINC filter notch at 8 kHz. Additionally, a similar effect can be achieved by running at a faster data rate and averaging the conversions results.

    Best Regards,
    Chris

  • Hi, Chris

    Thanks for the deep analysis. Sorry I do not quite understand "averaging the conversions results". And you mention placing a SINC filter notch at 8kHz, do you mean placing SINC filter outside ADS1282?

    Best Regards
    Yang
  • Hi Yang,

    Looking at the filter responses for 8 kSPS, 16 kSPS, and 16 kSPS with a moving average of 2 samples:

    ...The SINC filter has notches in the frequency-domain at integer multiples of the data rate. Therefore, by running the ADS1282 at the 8 kSPS data rate, you'll effectively filter out the 8 kHz noise.

    Additionally, if you choose to use a faster data rate (such as 16 kSPS), you can take a moving average of the data, which will also put notches in the frequency domain at integer multiples of the effective data rate. So taking a 2-point moving average of 16 kSPS, places a notch at 16 kSPS / 2 = 8 kHz.

    The moving average calculation would need to occur in your microcontroller or digital signal processor. Since a moving average filter is equivalent to a SINC1 filter,  yes, you would be creating a type of SINC filter outside of the ADS1282. :) 

    Here is a good reference on the topic of Moving Average (SINC1) filters:  

    Best Regards,
    Chris