Investigating the right leg, RL, A or GND electrode in ECG, EEG and other biosignal measurements.
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- čas přidán 12. 11. 2018
- N or RL or A1/A2/M1/M2 is the mysterious 3rd electrode when measuring biosignals such as ECG, EEG or EMG but do we need it? Yes we need it but it doesn't matter where we place the electrode. This electrode in technical terms is called GND and is there for pure technical reasons because amplifiers always measure against GND. However in biosignal applications GND is cancelled out by calculating the difference between two amplifier signals -- both measuring against GND. So it doesn't matter where the GND is placed on the body. However, it reduces 50Hz/60Hz mains interference and in the old analogue days GND actually injected an inverted mains signal in the body to eliminate it even more and people called it rather N for neutral. Nowadays the main interference can be filtered out by a digital filter as shown in the 2nd trace in the clip and much less effort has to be put into the analogue design which won't require perfect mains cancellation. One can even measure an ECG with an amplifier which measures against GND and then do heavy digital filtering to remove the mains interference.
biosignals.berndporr.me.uk - Věda a technologie
Biomedical PhD candidate here. Kinda best intro for RL drive. I was wondering it in real application and you did it here. You saved my time. Thank you and subscribed
I really impressed by your efforts and passion to deliver sophisticated knowledge in a simple and real way! thank you so much!
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Thank you for your clear explanation! It is really useful
This is awesome explanation ! Keep it up ! great work..!
Hello there, I am kind of confused at the end. Why does "two-electrode" EMG signal has more 50 Hz power line noise in it? Didn't we said that it actually has three electrodes as well since ground and the negative inputs are connected internally? So technically both channel 1 (ECG data) and channel 2 (EMG data) have 3 electrodes right? If so, I don't quite get what is the difference between first channel and second channel that causes one to cancel out the power line noise and other to not. I would be glad if anyone could help. Cheers. :)
Hey, I have question, if I want to record in lay down position, is it safety?
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Thanks for the vid. Pls, are you using a wet or dry electrode? Where can I get some of those components you displayed?
check out biosignals.berndporr.me.uk
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What about if I use single supply config. Do I connect ref voltage to negative input if i want 2 electrodes configuration.
Single supply config? You mean no negative power supply? ECGs have negative components so won't work. Either way it doesn't matter if you connect one electrode to GND or Ref. It's always a "ref" either to GND or ref voltage. What you get then is a simple non-inverting opamp circuit. GND to right arm. Non-inverting input to left foot. That won't cancel out 50Hz so you'd need a very good 50Hz mains filter. When using a wireless box (like the Attys) without a connection to mains GND that's not too bad when short circuiting GND and the neg input which is done for the 2nd channel in the Attys but needs a good digital notch filter to remove 50Hz. www.attys.tech/how-to/
If the grounding electrodes cancel out, why do amplifiers need a ground? Can you explain this?
Amplifiers need to measure against GND. There is no way to do that without it.
Is it a driven right leg or just neutral?
Neutral
Would you still be able to connect the two electrodes together if the "right leg" were driven?
@@ytubeleo Yes. The driven right leg is still basically "N" but injecting back a small amount the inverted amplifier output so it will still reduce 50Hz. However, the right leg drive is less and less used as it was an analogue means of reducing 50Hz noise and digital filters offer a cheaper way of doing so.
Some segments in the video are stamped not adjacent to each other
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