Are My ECG Filters Meeting the Industry Standard for Cuttoff Frequencies?

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Discussion Overview

The discussion revolves around the cutoff frequencies of filters used in an ECG system, specifically questioning whether the observed cutoff frequencies of 1 Hz for the highpass filter and 80 Hz for the lowpass filter meet industry standards, which are typically 0.1 Hz and 100 Hz, respectively. Participants explore the implications of these settings on signal quality and noise interference, as well as the technical aspects of filter design and noise sources.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • Some participants question how the waveform is distorted when using the standard cutoff frequencies of 0.1 Hz and 100 Hz, suggesting that sharing schematics and waveform captures could aid in analysis.
  • One participant notes that the significant reduction in noise with a small shift in cutoff frequency is impressive, attributing it to the use of an 8th order filter.
  • Concerns are raised about potential noise sources, such as 120 Hz from rectified power supplies, although one participant clarifies they are using batteries instead.
  • Participants suggest using MATLAB to calculate the FFT of the waveform to identify undesirable frequency components and determine appropriate filter coefficients.
  • There is a discussion about the challenges of avoiding 50 Hz noise and its harmonics, even in battery-powered circuits.
  • Some participants speculate on the rationale behind the standard cutoff frequency of 100 Hz, questioning whether it is tailored for regions with 60 Hz mains frequency.
  • One participant expresses uncertainty about whether an 80 Hz cutoff would be acceptable, emphasizing the need for their work to align with established references for their research paper.
  • Questions are raised about the number of leads used for ECG pickups and the importance of grounding in the circuit design.
  • Concerns are mentioned regarding the presence of harmonics in the power line waveform and their effects at the sampling rate of 400 Hz.

Areas of Agreement / Disagreement

Participants express differing views on the appropriateness of the cutoff frequencies for the ECG filters. While some support the use of 80 Hz as a valid alternative, others emphasize adherence to standard values. The discussion remains unresolved regarding the optimal cutoff frequencies and the implications of noise interference.

Contextual Notes

Participants highlight limitations such as the dependence on specific definitions of noise, the effects of harmonics, and the lack of consensus on the best practices for filter design in ECG systems.

samaaa
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Hi guys

i built an ECG system(preamplifier and filters), but i faced a problem in the filters:

the standard of ECG filters is 0.1 Hz cuttoff frequency for highpass filter and 100 Hz for lowpass filter

but i only obtain a good ECG signal when i make the cuttoff frequency of highpass filter 1 Hz and lowpass of cutoff frequency 80 Hz(8 order)

is my ECG filters correct(1 Hz and 80 Hz)?
or the filters cuttoff frequencies should be as the ECG filters standard?

i ask this question because i am writing a research paper.
 
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When the cutoff frequencies were at 0.1 and 100 Hz, how is the waveform distorted? If you can post schematics and waveform captures we can better analyze the problem.
 
skeptic2 said:
When the cutoff frequencies were at 0.1 and 100 Hz, how is the waveform distorted? If you can post schematics and waveform captures we can better analyze the problem.

ok ,this the ECG signal when the cutoff frequency of lowpass filter is 100 Hz
image.png

and this the ECG signal when the cutoff frequency of lowpass filter is 80 Hz
image.png
 
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The fact that the noise is reduced so dramatically by such a small shift in cutoff frequency is impressive and likely due to your use of an 8 order filter. First I would consider sources of 120 Hz such as the rectified power supply. Of course it may not be 120 Hz noise and it would help to expand the display to see if you can determine the frequency.
 
skeptic2 said:
First I would consider sources of 120 Hz such as the rectified power supply. Of course it may not be 120 Hz noise and it would help to expand the display to see if you can determine the frequency.

i am not using power supply , i use two batteries (of 6 volt)

how can i determine the frequency noise?
and how can i prevent it from Interfere with ECG signal?
 
samaaa said:
... how can i prevent it from Interfere with ECG signal?
Looks like you've done that. A schematic of the filters would be nice to see though.
 
dlgoff said:
Looks like you've done that. A schematic of the filters would be nice to see though.

i use digital filter(matlab) , my hardwere is only preamplifier.
 
Can you use Matlab to calculate the FFT of your waveform? That will give you a good idea what coefficients you should use for your filters.
 
skeptic2 said:
Can you use Matlab to calculate the FFT of your waveform? That will give you a good idea what coefficients you should use for your filters.

ok dear i will do that as soon as, thank you
 
  • #10
I agree. The FFT will indicate what undesireable frequency components may be causing the noise. But, not where they are coming from.

The next questions will probably be: What is the sample rate of the system, and what are you using for an anti-alias filter?

For the FFT use as many points as you can and experiment with the windowing function for the best results.
 
  • #11
In a region where the mains frequency is 50Hz, it is very difficult to avoid picking up 50 Hz (and harmonics) on body electrodes even if your entire circuit is battery powered and has no reference to the mains.
 
  • #12
skeptic2 said:
Can you use Matlab to calculate the FFT of your waveform? That will give you a good idea what coefficients you should use for your filters.

meBigGuy said:
I agree. The FFT will indicate what undesireable frequency components may be causing the noise.

this is the FFT of ECG signal
image.png

it very clear that the noise 100 Hz com from power line harmonic

meBigGuy said:
The next questions will probably be: What is the sample rate of the system, and what are you using for an anti-alias filter?
sample rate= 400

i am not using anti-alias filter,because the 100 Hz noise i can remove it in matlab

NascentOxygen said:
In a region where the mains frequency is 50Hz

yes the frequency of power line in my country is 50 Hz

NascentOxygen said:
it is very difficult to avoid picking up 50 Hz (and harmonics) on body electrodes even if your entire circuit is battery powered and has no reference to the mains.
No solution :confused:
 
  • #13
Why do you think the standard cutoff frequency is 100 Hz? Is the standard ECG designed for use in a 60 Hz country and the 100 Hz cutoff would then be effective for 120 Hz? I find it hard to believe that even when the heart is in fibrillation that you would have signals above 80 Hz. Can you think of any reason why an 80 Hz cutoff would not work?
 
  • #14
How many leads are you using for the EKG pickups on the body? Can you post your schematic (as has been mentioned already). Do you have a ground lead as part of your circuit (like below). It's good that you are just battery-powering your circuit!

http://www.cs.wright.edu/~phe/EGR199/Lab_1/fig3.gif

 
Last edited by a moderator:
  • #15
Oh, and I agree that your upper cutoff frequency should ratio lower since the 100Hz number is likely meant for countries that use the 60Hz AC Mains frequency.
 
  • #16
skeptic2 said:
Why do you think the standard cutoff frequency is 100 Hz?
I read in many book, the ecg lowpass filter is 100 hz cutoff frequency

skeptic2 said:
Why do you think the standard cutoff frequency is 100 Hz? Is the standard ECG designed for use in a 60 Hz country and the 100 Hz cutoff would then be effective for 120 Hz?
Yes, I think that

skeptic2 said:
I find it hard to believe that even when the heart is in fibrillation that you would have signals above 80 Hz. Can you think of any reason why an 80 Hz cutoff would not work?
i also see that 80 Hz cutoff work properly but as i mentioned i am writing a research paper, my work must be
agree with a reference book or with another research paper
 
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  • #17
berkeman said:
How many leads are you using for the EKG pickups on the body? Can you post your schematic (as has been mentioned already). Do you have a ground lead as part of your circuit (like below).
i use three leads ,one lead is grounded(right lead),
my schematic is like(similar) the schematic you put it:
http://www.cs.wright.edu/~phe/EGR199/Lab_1/fig3.gif

berkeman said:
Oh, and I agree that your upper cutoff frequency should ratio lower since the 100Hz number is likely meant for countries that use the 60Hz AC Mains frequency.

i agree with you too:smile:
 
Last edited by a moderator:
  • #18
The power line waveform is thought of as being a sine wave, but what if there are harmonics present? What happens to those harmonics at your 400 Hz sampling rate?

Also consider what happens if you are sampling at an integer multiple of the power line frequency.
 

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