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Recorded ECG with two GND electrode placement

The above figure shows two ECG signals recorded after placing the positive electrode on the left palm and the negative electrode on the right palm. For the recording on the top column, the ground electrode is placed on the leg, whereas for the bottom recording, the ground electrode is placed on the palm beside the positive electrode.

The recording with GND on the leg has a higher dynamic range (which might be good) but more high-frequency noise (which is bad) than the recording with GND on the palm.

Both recordings are collected with the ADI instrument's FE232 bio-amplifier with no right-leg drive circuit.

Theoretically, I expected similar signals independent of the placement of ground electrodes. In both cases, in the end, we are measuring differential signals, through the op-amp, between the positive and negative electrodes.

My questions are:

  1. How does the GND interact with a differential signal at the circuit level? What role does a GND electrode play in measuring biopotentials, not necessarily ECG?
  2. Why do I observe a voltage drop in the signal after placing the GND electrode closer to the positive electrode?
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    \$\begingroup\$ "Noise" is any signal that you did not intend to measure. What do you think happens when you have more components in series with something you wish to measure? \$\endgroup\$
    – Abel
    Commented Nov 13, 2023 at 13:40
  • \$\begingroup\$ @Abel. Do you mean that the voltage will drop if there are more resistive elements in series? I define noise in my context as a very high-frequency signal. \$\endgroup\$
    – hari
    Commented Nov 13, 2023 at 14:46

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The ground electrode of an ECG is never connected to the preamp ground because it feeds a lot of noise. Instead the common-mode noise is picked up from the instrumentation amplifier IC, inverted with an opamp and fed to the patient's right leg to cancel the noise. Here is the circuit with a AD620A instrumentation amplifier: ECG

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  • \$\begingroup\$ I agree. But, in both of my cases, I have the same electrode and circuit configuration, except for the placement of the GND electrode. Why do I see the drop in voltage after placing the GND electrode close to the positive electrode? I am not using the Right Leg Drive feedback on the GND electrode. \$\endgroup\$
    – hari
    Commented Nov 13, 2023 at 18:23
  • \$\begingroup\$ I do not know why the experts selected the right leg. \$\endgroup\$
    – Audioguru
    Commented Nov 13, 2023 at 22:03
  • \$\begingroup\$ A possible reason for selecting the right leg for GND is that it is the farthest point on the body from the source of the biopotential (i.e. heart). \$\endgroup\$
    – hari
    Commented Nov 14, 2023 at 6:07
  • \$\begingroup\$ The right leg is the farthest part of a person from the heart. Then the inverted common-mode interference is connected there to cancel the interference. Ground is not used on the person. \$\endgroup\$
    – Audioguru
    Commented Nov 15, 2023 at 13:20
  • \$\begingroup\$ As per your suggestion, one should use right leg drive feedback for ECG. There are a few home-use-based ECG devices in the market that do not have any GND connection to the right leg, such as Kardia from Alivecor. Could you comment on how they might work? \$\endgroup\$
    – hari
    Commented Nov 15, 2023 at 13:25

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