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(I do hope my question is not off-topic)

Currently learning about ECG recorders (Our 7-months old daughter has to have one during her sleep), I wonder whether this is a pure "passive" measuring device (like a multimeter) or not.

I do think I understood that it measures heart activity by comparing potential differences between 2 or more body-connected electrodes.

But I did not understand whether this works purely by getting signals from the heart and processing them or whether the device actively sends some signals through the electrodes and measures what comes back.

So my question is:

Does an ECG recorder send signals trough the electrodes or not?

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    \$\begingroup\$ now you made me want to see if its possible to build some myself... \$\endgroup\$
    – PlasmaHH
    Commented Nov 27, 2014 at 10:11
  • \$\begingroup\$ @PlasmaHH Post some pictures, when you're finished :-) \$\endgroup\$
    – Uwe Keim
    Commented Nov 27, 2014 at 12:30
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    \$\begingroup\$ googling for them returns already lots of pictures ;) \$\endgroup\$
    – PlasmaHH
    Commented Nov 27, 2014 at 12:35
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    \$\begingroup\$ I would like to point out that a muti-meter is NOT a passive device. Current (although small) has to pass through the probes to make a measurement. \$\endgroup\$
    – Guill
    Commented Nov 29, 2014 at 4:06

2 Answers 2

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An ECG recorder works by recording electrical signals produced by the heart, and does not need to provide any active signal to do so.

That said, most commercial ECG systems will actually use a driven ground arrangement, called a "driven right leg" to make the recordings better (feeding back the common mode signal through a reference electrode), and thus do "send signals through the electrodes" -- but those signals have nothing to do with the signals generated by the heart. From an electrical engineering point of view, this minimizes the effect of different impedances between the different electrodes and the skin, among other things.

There's a reasonable review of the method at http://www.ti.com/lit/an/sbaa188/sbaa188.pdf

As pointed out by Zayzoon there may also be a small current for detection of leads falling off.

All these currents are not dangerous, and you can use the device without fear of injury.

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    \$\begingroup\$ +1 A comparison might be touching a doorknob to get rid of a static charge except at about 1/100 the voltage and much less current. A properly working ECG is not a concern safety-wise. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Nov 26, 2014 at 19:44
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    \$\begingroup\$ 1/100th of the voltage of a 25,000 volt static charge is still 250 volts, I wouldn't find that very reassuring. Pushing 50uA through a 1000 ohm body would take around 50 milliVolts, much much lower than the voltage involved in static shock. \$\endgroup\$
    – Johnny
    Commented Nov 27, 2014 at 1:50
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    \$\begingroup\$ For more information, you can look at details of this part: ti.com/product/ads1293 , which is used in several commercially made ECGs. \$\endgroup\$
    – whatsisname
    Commented Nov 27, 2014 at 5:16
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Well, there is a good chance they actually do send a small current ~ nanoAmps range to make sure the electrodes are reliably attached to the patient as the measurements are meaningless if the electrodes are not properly attached. Depending on the design and who made it that circuit could differ.

But that is not related to how they measure electrical phenomena generated by the heart. basically an electrode is a sensitive capacitor which detects small changes of voltage generated by the heart as it beats. this induces a current flow within the electrode which has a very specific signature. it immediately gets digitized for signal processing purposes. I hope that helps. Zay

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