On an X Series device, you can either acquire a signal present on the I/O connector, or you can acquire a signal that is being generated from the internal calibration multiplexer. The channels available on this multiplexer are typically used for calibration purposes, but you can also sample them as you would a physical signal present on the I/O connector. To read from one of these internal channels on a X Series simultaneous sampling device, you must use one of the device's AI physical channels (for example, Dev1/ai0) when creating the virtual channel. The physical channel specifies the ADC for the internal channel. You can then set the appropriate string value on the Input Source channel attribute/property.