This voltage is dutifully amplified by the instrumentation amplifier and passed on as a generally saturating clipped sine wave to the rest of the system. This wouldn't be a problem, except that those signals pass along the length of the headbox cable along with the small signals on all of the other channels. This can overwhelm the smaller signals, causing fluctuations in measured impedance. Although this has not historically been a problem, it requires mention. The impedance measurement circuit assumes a full complement of connections to a subject for proper operation.
Good science dictates that the measurements obtained in that case be verified beforehand.
When making these measurements, it is important that the circuit characteristics be taken into account. Always use the ground connection. If the ground connection is left open during the measurement, currents from all channels will flow back through the series 1M Ohm resistors at the negative sides of the amplifiers. This causes errors as described above under the Bench Testing vs Subject Impedance Measurement. Short unused channels to ground for bench impedance testing.