What does a differential amplifier record?

A differential amplifier is designed specifically to amplify the difference between two input signals while rejecting any signals that are common to both inputs, such as noise or interference. This feature allows for a cleaner and more accurate recording of specific signals of interest, making the differential amplifier particularly useful in applications such as neurophysiologic monitoring.

When a differential amplifier processes two input signals, it computes the difference between them, which means that the output reflects only the unique aspects of the signal that are not shared with the other input. This capability is crucial in clinical settings, where clear detection of physiological signals is necessary amidst various background noises.

The other options do not accurately describe the function of a differential amplifier. It does not record the sum of the two inputs, nor does it only focus on the first input. Additionally, it does not measure the power of the difference; rather, it directly computes and amplifies the difference itself. Thus, the choice indicating that a differential amplifier records the difference between the two inputs is indeed the correct interpretation of its fundamental operation.