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 ?