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ADS1118-Q1: How to filter harmonics

Eileen Hu
Intellectual 1050 points

Part Number: ADS1118-Q1
Other Parts Discussed in Thread: ADS1118

Hi team,

My customer is using the ADS1118 to extract the 2nd harmonic of a 50 Hz fundamental signal. However, the input signal contains interference from the 0.5th harmonic, 3rd harmonic, 5th harmonic, and the fundamental component itself.
The customer observed that reducing the sampling rate from 860 SPS to 180 SPS suppresses these unwanted harmonics.
We would like to understand:
1. What is the principle behind this improvement?
2. Are there any risks or drawbacks to lowering the sampling rate?
3. Do you recommend any other methods to better extract the 2nd harmonic while rejecting other frequency components?
4. Are there any preferred MCU code or digital signal processing algorithms you can suggest?
Your technical support and advice would be highly appreciated.

Regards,

Eileen

  • 0
    TI__Intellectual 1900 points

    Hi Eileen, 

    Thank you for the questions. 

    1. By lowering the sampling rate the customer is reducing the Nyquist frequency (half the sample rate) which naturally attenuates frequency components above this threshold. This reduction is likely suppressing the higher harmonics more, relative to the desired 2nd harmonic, improving the signal to noise ratio for the target frequency. (See Section 10.1.3 of the datasheet) 

    2. Drawbacks could include reduced measurement precision with the lower sampling rate it can reduce the accuracy of the 2nd harmonic measurement itself. This can limit the ability to capture rapid changes in signal, potentially missing transient events or increasing the latency time in real-time applications. Using the 180SPS would also attenuate the target signal which may reduce the signal amplitude and require gain compensation to maintain accuracy. 

    3. Using a analog input filer could help to attenuate the unwanted harmonics. A simple 1st order RC low-pass filter (See Section 10.1.3 of the datasheet) could be a good hardware solution here to attenuate the higher harmonics. A passive band-pass filter centered at 100Hz could be used to attenuate both the lower 0.5th harmonic and the higher harmonics as well. 

    4. One option for digital post processing could be implementing a FFT (Fast Fourier Transform) to analyze frequency content to identify the 2nd harmonic amplitude. The Goertzel algorithm could be another option here due to its rather lightweight signal frequency detection, with multiple examples online for implementation. 

    Regards, 

    Andrew