Which device converts an analog signal into a series of digital binary numbers?

The A/D converter, also known as an analog-to-digital converter, is the device designed specifically to convert an analog signal into a series of digital binary numbers. This process involves sampling the continuous analog signal at discrete intervals and then quantizing these samples into binary values that represent the amplitude of the original signal at those points in time.

In various fields, including electroencephalography (EEG), the conversion of analog electrical activity into digital format is crucial for processing, analyzing, and storing the data. The ability to transform these signals into a binary format allows for easier manipulation and analysis using computer systems, enabling advanced data processing techniques.

Other options serve different functions. For example, a differential amplifier enhances the difference between two input signals but does not perform conversion to digital. A fast Fourier transform is a mathematical algorithm used to analyze frequencies within a signal but does not involve signal conversion. An impedance meter measures the opposition to current flow in a circuit, also not related to converting signals into digital form. Thus, the A/D converter stands out as the correct answer.