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Well, a colleague after all :-). More to agree than to diverge.Sorry to hear about you lost Windows patent. What a shame. How would you name your company? (When i think of it, I do remember an old Mel Brooks movie. The evil was called "Bite and Devours" or something like that.) Just joking. Dont take me wrong. By the way, i am from Argentina. If somehow i do manage to go to Switzerland (very unlikely, i am afraid), let me buy you a beer.You are right. Is getting too long. I am getting too old too... I must shut up.Best regards.
Well. I must agree with you... mainly. Calling it 'fake ground' was just a simplistic way to kickstart the original question. But, I must quote Wikipedia:"Leads aVR, aVL, and aVF are the augmented limb leads. They are derived from the same three electrodes as leads I, II, and III, but they use Goldberger's central terminal as their negative pole which is a combination of inputs from other two limb electrodes".Being picky, you said "Electrode 2 (left arm) (ie the heart as viewed from left shoulder - where the left arm joins the body!), using Electrode 1 as reference and Electrode 3 as a ground".Speaking strictly, we measure an electric potential against a reference, being this reference a simple one for bipolars (the other limb), or a compound reference for augmented ...see more »Well. I must agree with you... mainly. Calling it 'fake ground' was just a simplistic way to kickstart the original question. But, I must quote Wikipedia:"Leads aVR, aVL, and aVF are the augmented limb leads. They are derived from the same three electrodes as leads I, II, and III, but they use Goldberger's central terminal as their negative pole which is a combination of inputs from other two limb electrodes".Being picky, you said "Electrode 2 (left arm) (ie the heart as viewed from left shoulder - where the left arm joins the body!), using Electrode 1 as reference and Electrode 3 as a ground".Speaking strictly, we measure an electric potential against a reference, being this reference a simple one for bipolars (the other limb), or a compound reference for augmented (the other two limbs) or precordials (the three limbs of the Einthoven's triangle). Not a ground. Well. I do know. A ground lead most often does exist, but is connected to equipment's ground. On some (weakly designed) electrocardiograph, is used to improve mains hum rejection.Quoting Wikipedia again: "The precordial leads lie in the transverse (horizontal) plane, perpendicular to the other six leads. The six precordial electrodes act as the positive poles for the six corresponding precordial leads: (V1, V2, V3, V4, V5 and V6). Wilson's central terminal is used as the negative pole" and "The common lead, Wilson's central terminal VW, is produced by averaging the measurements from the electrodes RA, LA, and LL to give an average potential across the body".Wikipedia said things better than i can.On a personal note, I am a electronic hobbyist, from more than 40 years now. Yes. You are right, I'm not an engineer (surely you are).I developed this same thing for personal amusement more than 25 years ago, with opamps, and it was a pain, and a pleasure. The output was connected to an osciloscope with no persistence, so it was a pain to read. A trigger signal was also sent to a Commodore 64 to read heart's rate. Changing from bipolar to augmented to precordials was done with pushbutons. It was just fun and not for real use, even if it worked OK.But also I am an ICU M.D, for more than 30 years now. I can tell you first hand the huge difference between a weak or good designed electrocardiograph when you are in a RCP, massaging a heart, with all this movement interfering the readings, and you do need to distinguish what is happening inside the chest. And close enough is not enough when it defines if someone will live or die. (Yes. It is embarrasing. Is V1-V6, not V1-V5.)So, in my own opinion, for what is worth, for precordials you need what is properly called Wilson's central terminal.Ok. This is Instructables and i am being too much picky. But someone was enthusiastic about expanding this to be used on the field, so i think is better to say thing right.Anyway, you spotted a very weak design point: no low-pass filter. For any serious use is a must have.I enjoyed your other post. Maybe the article you was unable to find was "DIY EEG (and ECG) Circuit" by cah6, here at Instructables?. The link you posted "DIY ECG with 1 op-amp" is amazingly simple!Also, i do agree. 5v (or 12v) will do nothing to you (or me, or anybody). But discovering a just-starting PSU leakage when you connect your ECG machine to a patient can be... well... shocking. Because disaster strikes when no one expect this. So, for safety reasons, is better to stick to a battery source. Not because 5v will kill you. Because a PSU can fail after some time of use, and this kind of thing cant be prevented. (Safety First. A principle by which to live)By the way: I'm lucky enough to be NOT ruled by Mr Trump :) (I hope you do not either). But we live in the same planet. This is unlucky. Whatever happens on USA, repercutes on the entire world.. :(Be well.
Yep. V1-V6. My bad. The circuit that you describe is called aVL, the "a" stands for"augmented". They are unipolars, like Vx, and they fake ground against the other arm/foot (Same thing for aVR, aVF.). Moving the electrode to the six points on the chest can give you readings, but they will be better with the "fake ground" circuit. Making medical diagnosis is a tricky thing, and it depends heavily on the quality of the signals. A little up or down of the baseline and you can do the wrong diagnosis. Other than this, i do agree with you.Best regards
If i do remember well, V1-V5 are measured against a fake ground composed of left hand, right hand and left foot connected through 1Mohm resistors in a star configuration. The center point is the fake ground. Hope this helps. (A bit of googling will help too... ;) (Image stolen from Electrocardiografia.es)
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