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FFT and 60 Notch filter software

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ataradov:

--- Quote from: loop123 on March 12, 2024, 12:05:09 am ---Besides FFT. How to know what frequencies cause the interference or noises?

--- End quote ---
What is wrong with FFT? It is perfectly adequate.


--- Quote from: loop123 on March 12, 2024, 12:05:09 am ---It is said enclosing the whole setups in Faraday Cage can eliminate the distortion. True?

--- End quote ---
Depends on the source of the distortion. If your amplifier is crap, no matter how much you enclose it, it will remain crap.

I'm done with this this topic. I mistakenly did not ignore it from the start.

BrianHG:
Just so you are aware, if you are making sampling tech which needs to notch out 50hz and 60hz, if I remember correctly, Analog Devices makes a 24bit ADC which has an absurd built in 21 tap, or more, digital filtering processor specifically designed for that task to deal with AC emi and it's overtones.  It had something like an insane 180db reject on those 2 frequencies.

I once designed scales for food filling machines which had to sample really fast, yet reject the food feed vibration beds which were controlled by the my electronic scale.

It just requires 1 crystal and it provides you a clock with a fixed sample rate.  You can gang them together using only 1 crystal to create a huge channel sampling system with channel to channel perfect reject of those tones.

loop123:
The amp (model AMP01) used in the commercially built equipment is not crap. It is one of the best with noise of 5 nV/sqrt(Hz) so at 100 Hz bandwidth, the noise rms is 50nV or 0.05uV so 10uV signal should have no problem. If I increase the signal to 30microvolt, 50Hz. The following is nice sine wave output with no distortion (using 100Hz bandidth too in amp). It is only when I used 10microvolt that I got the distortion. Using the 10uV wav file I shared in last message. Guys pls help determine the frequencies interfering with it. From the file, can you tell if the noise is external or in the circuit??

SiliconWizard:

--- Quote from: BrianHG on March 12, 2024, 12:19:47 am ---Just so you are aware, if you are making sampling tech which needs to notch out 50hz and 60hz, if I remember correctly, Analog Devices makes a 24bit ADC which has an absurd built in 21 tap, or more, digital filtering processor specifically designed for that task to deal with AC emi and it's overtones.  It had something like an insane 180db reject on those 2 frequencies.

--- End quote ---

Yes. They have many of them actually, for instance the AD7190.

Andy Chee:

--- Quote from: loop123 on March 12, 2024, 01:19:50 am ---From the file, can you tell if the noise is external or in the circuit??

--- End quote ---
Are you only measuring one channel/electrode, or multiple channels/electrodes?  Your photo only shows one waveform, so I'm assuming you're only measuring a single channel/electrode.

If you measure multiple electrodes, you can mathematically remove the common noise.  The theory is that the skin potentials will be different at each electrode, but the noise (particularly 60Hz and other EMI) will be the same in all channels. 

EEGlab can do this multiple channel common noise reduction, and I'm pretty sure Audacity should be able to do it with two channel stereo.  The key is that you need multiple channels, not single.

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