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I am trying to read the values from a simple EMG circuit (with an INA122P) using the analogRead() and plotting in the plotter using an Arduino nano v3. I am powering the ina1222p from the 5v pin of the Arduino. But no matter what I do, the signal comes out very noisy most of the time(i can rarely make out the EMG). The bandpass filter is also not really helping. I am using regular solid core wire and alligator clips to connect to the electrodes.

Can someone please tell me what I am doing wrong? or tell me how to make a simple EMG circuit that uses the Arduino's analogRead().

I am also getting ac sine waves when my laptop is plugged to power, so I tried removing the power. The distinguishable sine wave wasn't really showing up, but it was still just as noisy...

I am a beginner to both Arduinos and designing Analog circuits.

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I found this design from here: https://greenhecktech.wordpress.com/2016/09/19/improving-the-emg-amplifier/

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  • If you could provide a schematic that would really help! One thing you might quickly be able to try is running from a powerbank. You also want to make sure you decouple the power rails of the INA122P with (preferably both) a 10nF and a 100nF capacitor. – Ananas_hoi Sep 21 '20 at 15:59
  • @Ananas_hoi a powerbank could have an electrically noisy buck converter ... try to run from a battery directly – jsotola Sep 21 '20 at 16:17
  • You are right. Furthermore, soldering the connections would be desirable over a using a breadboard. It might also help to keep the connections short and isolate wires from noise with some simple aluminium foil shielding. Since it's a dual input amp, consider looking at this: toshiba.semicon-storage.com/us/semiconductor/knowledge/faq/… Without schematics I cannot help any further. – Ananas_hoi Sep 21 '20 at 16:25
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"noisy most of the time" - good news: if it's not all the time, that's a sign it can be fixed.

"bandpass filter is also not really helping" - INA122P circuit likely has something wrong. check any resistors involved. Edit: if this was about U2.2, see below as that doesn't look like a band pass...

"regular solid core wire and alligator clips to connect to the electrodes" - basically 0 shielding with mini-rabbit ears at each end and a big loop area between the wires. Eventually you will want something better than gator clips. My main recommendation is that as far as wire goes, if you form twisted pairs they tend to be pretty darn good at rejecting noise at least for that part of the wire. You will want to twist a signal along with its reference (such as a +V and a ground).

Some explanation of the circuit:

U2.1 is creating a 2.5V source that it uses as the reference 'ground' for U1. Reason for this is to treat 0 as "-2.5" and 5 as "2.5" as op amp math is generally done on a +V, -V basis. (Personally, I prefer to use voltage regulators to make power buses instead of this. As a result, I give a stink eye to circuits that use the op amp method and claim something about reducing part count.)

U2.2 is a capacitor shy of an active band pass. It's a high pass filter that is helping preserve and even promote some of the high frequency noise you are picking up. So yes, you may wish to modify or even ditch it until you have high frequency noise under control.

If possible, you may also wish to look into operating at a higher voltage. This can help improve your signal to noise ratio. If your signal is essentially +/-2.5V, 250mV of noise would be 10:1, but if you can run at +/-5 V, that same 250mV noise contribution gets dwarfed 20:1.

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  • i'll try using a higher voltage as you suggested. I have a question...why does this circuit use an opamp to provide a reference voltage?, i've seen other circuits use just the resistor network(r2, r3) to do that... what benifits does the opamp provide? – Plutonian Fairy Sep 28 '20 at 11:51
  • op amp or voltage regulator can provide current without voltage drop. The resistor voltage divider voltage changes as you draw current because another "resistor" path is added to ground, making the equivalent resistance change. – Abel Sep 29 '20 at 1:43
  • Ah, I got it, thanks~ – Plutonian Fairy Oct 2 '20 at 10:51

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