A to D is the abbreviation for Analog to Digital Conversion. Once the signal has been amplified and filtered it sent to the Ato D converter as well as the oscilloscope. The coaxial cable connects to one of the 16 available channels on the Ato D connector board. The channel is specified in the acquisiton program. The rate of conversion (sampling rate) and the digitizing range (voltage range) are also under software control.