About bessel filter on ECG/EMG shield schematic

[akif_rahmatillah]

We have a research in myoelectric proporsional control and we are considering to use ECG/EMG signal. But, when we saw the schematics, we saw the "besselworth" filter is desinged for cut off freq at 40 Hz.

1. Is the funtion of a "besselworth" filter for remove a noise from power supply (harmonic frequency 60Hz or 50Hz). Can we use for power suply frequncy in 50Hz ?

2. If we need to get myoelectric signal from 5 muscles that mean we need 5 shields

Thanks advance for your help

Akif Rahmatillah

[LubOlimex]

Hello, akif_rahmatillah

1) Yes, you can use power supply for 50Hz and/or 60Hz utility frequency. The filter cuts everything over 40Hz.

2) Depends if the muscles are in a group of muscles or not. If they are in the same muscle group one shield might be enough if you detect the levels of all possible variations. Please check this project: http://olimex.wordpress.com/2013/06/19/amazing-project-with-shield-ekg-emg/.

Edit:

1. The last filter does not filter mains electricity hum, for sure. You would get 50Hz hum (if in Europe) and 60Hz hum (if in USA) that you have to filter yourself.

The filter takes effect at 40Hz indeed, however the filter does NOT cut everything over 40Hz. This last filter has a 19.2dB rolloff per octave.

I highly recommend you the document that we used as inspiration for the shied: http://openeeg.sourceforge.net/doc/modeeg/modeeg_design.html - especially the part "A. Filter specifications" (of course, the EEG-EMG has higher cut-off frequency at 59Hz, which in SHIELD-EKG-EMG is 40Hz).

Best regards,
Lub/OLIMEX

[hjmhdr]

Dears,
As you know the range of the EMG signals are between 10-600 Hz. So by using the "basselworth filter 40Hz" the signals over 40 Hz cut off and a large part of the signals is neglected.
How it is possible to remove this filter from the EKG-EMG Shield or is there other alternative to acquire the whole signals?
TNX.

[LubOlimex]

Hey hjmhdr,

Please note that we don't encourage hardware modifications and doing such you would lose your warranty!

Also, please, consider that if you remove this filter there would be strong interference from the input power, so you should make make custom filtering of the supply.

That being said you can remove the 40Hz filter if you remove resistors R14 and R18 and close the pads by connecting the pads with a wire accordingly. Note that the pads you should connect are the ones on the outer side of the Bessel filter. Please check the schematic for exact positions (use the search for R14 and R18): https://www.olimex.com/Products/Duino/Shields/SHIELD-EKG-EMG/resources/SHIELD-EKG-EMG-REV-B-SCHEMATIC.pdf

Again keep in mind we don't encourage hardware modifications, these might brick the your device!

Best regards,
Lub/OLIMEX

[hjmhdr]

Thank you so much for your help.
I didn't modified the hardware and acquired the muscle signal. I attached the FFT graph and the signal graph to this message.
According to the figures, we have the signals more than 40 HZ, despite there is a besselworth filter in the shield.Now I want to know that, is the acquired signal true or I made a mistake?
If true, why the filter didn't cut off the signals more than 40 Hz?
Thanks.

[hjmhdr]

Thank you so much for your help.
I didn't modified the hardware and acquired the muscle signal. I obtained the FFT graph of the signals and I  have the signals more than 40 HZ, despite there is a besselworth filter in the shield.Now I want to know that, why the filter didn't cut off the signals more than 40 Hz?
Thanks.

[LubOlimex]

Hey,

For what amplitude is that? For small amplitudes it would be normal.

Best regards,
Lub/OLIMEX