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I was at my doctor's office and noticed an EKG machine that looked like a standard oscilloscope. In fact, its case was almost identical to my dual channel Tektonix 1104b. The main difference was that it had more probes, but I didn't check if they connected to different channels.

Can I, for instance, use my a scope as an EKG? If so, would it need any modifications?

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    \$\begingroup\$ It will need front-end/amplifier for EKG. \$\endgroup\$
    – jay
    Sep 21 at 17:14
  • \$\begingroup\$ what about # of channels? \$\endgroup\$
    – user148298
    Sep 21 at 17:18
  • \$\begingroup\$ I remember seeing some interesting waveforms when I touched the leads of those cheap solder together scopes. I wonder what the waveform was. \$\endgroup\$
    – user148298
    Sep 21 at 17:19
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    \$\begingroup\$ Probably 50Hz (or 60 in some countries) mains hum (electric field) picked up by your body. \$\endgroup\$ Sep 21 at 17:22
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    \$\begingroup\$ Ha! here, the electronics. There was a similar discussion, once I engaged. You can search it on StackExchange. \$\endgroup\$
    – jay
    Sep 21 at 17:27
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Absolutely, yes. But you'll need a frontend.

An EKG is usually just an instrumentation amplifier or a multistage amplifier with as many dB of common mode rejection as you can get.

The problem is that people are bags of electrolyte-rich fluids and meat with that right combination of resistance on the outside (skin) and relatively lower impedance to AC signals that all sorts of radiated EMI couples into our skin all the time.

If you've ever accidentally touched the tip of a speaker jack with your finger, or heck, just try probing your finger with your oscilloscope (isn't that the first thing one does with a new scope? I know it is for me!), you'll see what I mean. There is probably a volt or 2 of 50Hz or 60Hz ripple just from the wiring in your house.

This presents a problem when trying to detect electrical signals originating deep inside the skin-enclosed meat sack with all that noise coupling right at the surface trying to drown it out.

Where that noisy garbage coupling into your skin is on the order of a volt or two, the signals from your heart (at the point of detection on your skin at least) are about 1mV peak to peak. To get a good waveform, you'll want to be able to clearly see peaks as low as 20µV.

So you can't use an oscilloscope unaided, you can certainly use one as a very effective ECG/EKG with a front-end.

Such a front-end is not particularly difficult to make. The bandwidth and frequency of signals from the human heart are... quite slow in the context of analog electronics. This makes a frontend particularly forgiving, and you can even build one on a breadboard with just one IC.

Essentially, it all boils down to two things: You need a lot of gain, and a lot of common mode rejection.

Common mode rejection is achieved by, at the simplest, coupling the op amp's ground reference to your own body through a low (but not too low) resistor, like 100Ω. This is the lead that tends to connect far away from your chest, like at the angle or leg. This ensures only the noise gets picked up and rejected as common mode, leaving the cardiac signal (which are far too weak to matter that far away in your ankle or where ever).

A higher performance way of achieving this is to actually use a second op amp to drive the ground reference (your skin) and actively cancel out most of the common mode noise.

If you search for 'ECG frontend circuit', you'll find quite a few complete schematics of varying simplicity or complexity.

Any of them can work, but it ultimately depends on what level of performance is acceptable.

I have personally made this one, and a different similar one that also used a right leg driver that I can't seem to find, and both exceeded my expectations. They worked quite well:

enter image description here

Safety Note

Do not attach anything connected to your body like this to an oscilloscope that plugs directly into the wall. You must use an isolation transformer to fully isolate the oscilloscope so no ground path can be taken through you (and probably the electrical conduction system of your heart). Real ECGs are fully isolated if they plug into the wall at all.

Final note: Your body doesn't run on electric currents.

It runs on ionic current. Electric current has electrons as the charge carriers making up that current, but inside your body, it is positive ions rather than electrons that flow and do things like make your muscles move.

For that reason, you need to use some sort of electrolyte between an electrode and your skin, forming a half cell and allowing the ionic currents to be converted into electric currents for use with your ECG frontend.

Buying proper medical electrodes is ideal, but I can personally attest that tinfoil with some shampoo smeared on it can work in a pinch if you are really impatient to test what you made. Your results may vary.

Use an isolation transformer and don't get hurt or killed accidentally. Beyond that, this is actually a great project that isn't too hard or expensive to build if you're sufficiently interested and motivated. Good luck!

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    \$\begingroup\$ “Do not attach anything connected to your body like this to an oscilloscope that plugs directly into the wall. You must use an isolation transformer to fully isolate the oscilloscope” Why? You touch the probes and knobs of an oscilloscope all the time. Oscilloscopes which electrocute their users would be quite unpopular. \$\endgroup\$
    – Michael
    Sep 22 at 12:43
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    \$\begingroup\$ @Michael electroscopes sometimes fail, like any other electrical equipment, and can shock user, just like anything else can in case of malfunction. If you touch it by hand and current goes to your feet, it hardly touches your heart. Unpleasant, harmful, but usually easily survivable. But if you have two electrodes with heart directly between them, then such accident would run current straight thorough your heart, where it has the biggest chance to kill you. Well, brain might be worse, but that's it. \$\endgroup\$
    – Mołot
    Sep 22 at 14:44
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    \$\begingroup\$ This is a brilliant answer. It nearly made me go and build such an amplifier, and I don't remotely need one. \$\endgroup\$
    – 2e0byo
    Sep 22 at 18:18
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    \$\begingroup\$ I would add as an extra warning that one must not use an oscilloscope that is connected to anything that plugs into the wall unless the EKG front end includes a safety-rated isolation amplifier. I'd also emphasize that there is a huge difference between the level of shock one gets touching something with dry skin, versus a connection with an electrode that's deliberately designed to maximize skin conductivity. \$\endgroup\$
    – supercat
    Sep 22 at 20:31
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    \$\begingroup\$ @Michael Touch the terminals of a 9V battery to your finger. Now touch the terminals to your tongue. When you've stopped saying "ouch", you'll appreciate that what's fine to touch with your fingers may not be a good plan for more conductive parts of your body. \$\endgroup\$
    – Graham
    Sep 23 at 10:58
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You can find very simple integrated front-ends which perform the necessary signal conditioning. Just as an example: AD8232

enter image description here

Also available in boards:

enter image description here

Source: sparkfun

Another (harder) option would be a custom circuit based on an instrumentation amplifier to remove the DC offset and perform the filtering required for your oscilloscope to correctly acquire the signal.

Edit: to provide some extra information on the difference between electrodes and leads:

Wikipedia article on Electrocardiography

ecgwaves.com

Basically you perform differential measurements providing a reference (usually to the right leg) to improve baseline removal.

12-lead ECG:

  • 4 Electrodes on all 4 limbs (RA, LL, LA, RL)
  • 6 Electrodes on precordium (V1–6)
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  • \$\begingroup\$ This is not a beginners circuit, go with the pre-made PWB. \$\endgroup\$
    – Mattman944
    Sep 22 at 1:26
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    \$\begingroup\$ When you go to your doctor's office they typically use many leads. Up to 12. This board is for a 3 lead EKG, and this is a common way that portable EKG monitors (aka Holter monitors) work. It's also worth pointing out that to use the sparkfun board, you also want to purchase/make a special three-electrode cable assy that has the connector to the board on one end and 3 snaps on the other end. These boards are great and I own a few. \$\endgroup\$
    – bfris
    Sep 22 at 16:26
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    \$\begingroup\$ @bfris This is a one lead front end. When a medical technician talks about an ECG 'lead', they mean a pair of pads for heart potential, and possibly a leg drive as well. Some pads can be shared between 'leads'. So it's a one lead, three electrode, front end. \$\endgroup\$
    – Neil_UK
    Sep 24 at 14:25
  • \$\begingroup\$ So how many channels do you need for say 12 leads? Are the signals mixed together? Pardon my layman's parlance. \$\endgroup\$
    – user148298
    Sep 24 at 16:38
  • \$\begingroup\$ @user148298 I've added some references to the answer \$\endgroup\$
    – devnull
    Sep 24 at 17:45
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An EKG machine does have a display not unlike an oscilloscope. That's clear enough.

But what an oscilloscope is missing is the detection and amplification circuitry that pick up the heart's electrical impulses and amplify/condition them appropriately so that they can be displayed on a screen or plotted on paper.

You could build an EKG front-end for your scope and have a (likely) crude EKG-like device.

I did find this article on how EKGs work and an example circuit that might help you if you want to pursue this more:

EKG

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