0
\$\begingroup\$

My question is related to the number of electrodes used in ECG/EMG kind of signals. I know that voltage measurement is a potential difference measurement. I understand the concept of signal and reference. So naively I can think of 2 electrodes. But most of the first stage in detectors is instrumentation amplifier which can help in deleting the common mode signal. But when you measure using instrumentation amplifiers you need 2 input signals.

So actually are we measuring the difference of two ECG/EMG signals from body (at different locations) using 3 electrodes ?

\$\endgroup\$
  • 1
    \$\begingroup\$ One of them is Earth ground. \$\endgroup\$ – Tim Spriggs Apr 2 '16 at 18:16
3
\$\begingroup\$

In fact, two is enough. Try putting an electrode to each hand. Yet, some ECG input stages have almost literally infinite input resistance. So just two electrodes may (and actually do) float away out of the input range. So here comes the third electrode, that puts the whole body potential somewhere near the 0V of the input.

By the way, the high input resistance is required to maintain best possible CMRR, which is mandatory for good ECG reading.

Edit

Oh, i forgot another, even more common usage! Poor me... Thee is a feature called RLD. Similar to the reference i mentioned, but instead 0V it applies opposite of common mode from the two electrodes. That almost cancels the common mode, and again, improves CMRR.

| improve this answer | |
\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.