Many electrical biological signals are relatively low frequency. Outside sources of noise, such as that from the power grid and patient movement, can have higher frequencies than these signals and affect accuracy of signal reading. Low-pass filters can make use of active circuit elements, passive circuit elements, or both to attenuate undesired noises with higher frequencies and even amplify your desired signal. Using low-pass filters can help you get the signal you are looking for without all the excess noise! Watch the video below to learn how low-pass filters are designed, how they work, and how they can be used in biomedical instrumentation applications.
Self-Assessment Questions:
Click here to test your understanding of Low-Pass Filters.
Discussion Questions:
- The video discusses the application of ECG signal measurement for filtering of higher frequencies. The frequencies that make up ECG signals can range from 0 to 150 Hz. What could be some issues with using a low-pass filter to filter out 60 Hz noise from the ECG signal? How could these be combatted?
- Biological signals are not only collected with significant noise, but many signals such as ECG data, have very small amplitudes. Would you want to use passive low-pass filters, active low-pass filters, or both to obtain a clear, discernible signal? How would you set up these filters in your circuit?
- Other than an ECG signal, find at least three other electrical biological signals with low frequency ranges, determine appropriate cutoff frequencies for noise above this signal range, and draw a circuit diagram with components that you would use to implement this filtration strategy. Use this article by Renuka et al. to help identify other low frequency biological signals.
- The video stated that in an ideal low-pass filter, all frequencies above the cutoff frequency are attenuated. Why is this not the case (both scientifically and practically) in the real world? Why is there a slow attenuation after the cutoff frequency is exceeded? Use this article by Renuka et al. on filter principles to help your answer.
Continue Reading:
- Renuka et al., “Active Low Pass filter for biomedical applications,” International Journal of Engineering Sciences & Research Technology, vol. 6, no. 12, pp. 214–223, 2017. Read the article here.
- This article details the use of active and passive low pass filters in biomedical applications and introduces the use of CMOS integrated circuits for this purpose.
- J. Sun et al., “A low-pass filter of 300 Hz improved the detection of pacemaker spike on remote and bedside electrocardiogram,” Chin. Med. J. (Engl.), vol. 132, no. 5, pp. 534–541, 2019. Read the article here.
- Pacemaker ECG signals vary slightly from traditional ECG signals as they have a small “pacemaker wave” spike right before the P or QRS wave. This article details the results of a study that determines how to adjust low-pass filter cutoff frequencies to enable detection of this pacemaker wave.
About the Creator:
This video was created by a student in the Rice Bioengineering Class of 2023 (used with permission).
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