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Topic: Electromyogram problem (Read 2726 times) previous topic - next topic

claudiu_mirescu

Dear Arduino friends,
I am trying to make an electromyography project these days and I have a little problem. I made an EMG schematics using a simple AD620 amplifier and, when connected to the soundcard or the oscilloscope, it works very fine. It shows up and down waves when I contract the investigated muscle. But when I connected the same input to one analog input pin on my Arduino, it dows not work at all. When started, it gos from 1023 down to 0 regardless of the muscle contraction and it stops there, not reacting anymore. How could I remediate that, it is very annoying to now that your circuit works, but it cannot be read. I also posted a picture of the EMG circuit (imagine the electrodes placed on a muscle, not on the limbs, like for ECG) and a capture of the EMG signal collected with a soundcard. It works like that, but not on Arduino.

I would appreciate very much any suggestions.

claudiu_mirescu

I forgot, this circuit also works fine as an ECG on a scope or soundcard, but, again, on Arduino doesn't.

RIDDICK

the arduino can do 0V..5V...
and ur amplifier output seems to use a charge pump that produces -6V to 6V... via a quite fat 1mF cap...
maybe that much charge damaged the protection diodes of the arduino or the arduino input...

u could try to connect the arduino's ground to -6V... (hopefully it is a battery?)
and then use a voltage divider that brings the voltage below 5V...

what does the oscilloscope say about the peak voltage?
-Arne

Magician

You have to create DC offset, look at the right side:
http://coolarduino.files.wordpress.com/2012/06/audio_input.jpeg
BTW, what is the time / per division scale on the chart?

claudiu_mirescu

Ok, thank you for both answers. Peak voltage is about 3V. I do not know the exact scale of the chart, but on x scale are a few seconds and, as I said, the peak voltage is around 3V.

What if I use a rectifier bridge on output of the myograph?

Magician

A lot depends on what info you need from the pulses? Magnitude, freq. of sub-carrier, period of pulses, magn. both negative - positive, decay time etc.

RIDDICK

a rectifier bridge would work,
if u dont share the GND between the arduino and ur amplifier,
and if the absolute values r sufficient (the negative parts would show up as positive...)...


why cant u connect arduino ground to the negative supply rail of ur amplifier?
then u just need a voltage divider...
-Arne

claudiu_mirescu

I will try the second, connecting the negative to the ground and tell you if it works.

But if I were to use a bridge, where should I connect the GND of the myograph, if not to the GND of the arduino? I did such last night and it did not work.

Thanks!

RIDDICK

that bridge has a negative output, which can be used as arduino ground...

did u use a load resistor at the output of the bridge?

like this:
http://www0.wgboome.org/bridge.png
-Arne

claudiu_mirescu

No, I did not use a resistor, but, hopefully, my arduino pin didn't burn. It looks ok, I teste it with a potentiometer. I will retry ith the bridge and use the negative of the bridge to connect to the arduino GND. I'll get back to you soon.

RIDDICK

without a load resistor the pin will b always charged, because the ADC discharges it very slowly...
-Arne

claudiu_mirescu

And by that, you mean that the value on the pin won't change or it will change randomly? That's what happened to me and I didn't know why.

RIDDICK

-Arne

claudiu_mirescu

This software you are using when making the screenshots is available online? Could you send me a link?


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