1.4.4 Internal Input

With this setting, the MUX measures one of several internal signals. The negative input is always connected to GND while the positive input can be connected to one of the following internal sources: Temperature Reference, DAC Internal Output, AVCC/10 (for supply voltage measurement), or Bandgap Reference.

Note:

Two channels can select different internal sources. They are not limited to one common setting, as opposed to the shared gain stage setting.

The internal DAC input can be used for calibration of the DAC. For more information about DAC configuration, refer to the device data sheet or the “AVR1301: Using the XMEGA DAC” application note.

The Bandgap Reference can be used as a reference to calculate the external reference, like a battery voltage, if the voltage is unknown. With a measurement of a known voltage (the Bandgap Reference, 1.1V) using an unknown reference, it is easy to calculate the actual voltage of the external reference.

Figure 1-7. Internal Input in Signed Mode
Figure 1-8. Internal Input in Unsigned Mode
Note: If no other modules are using the Bandgap Reference it must be turned ON using the Bandgap Enable bit (BANDGAP) in the Reference Control register (REFCTRL).

The same goes for the Temperature Reference, which is not shared with any other modules. The Temperature Reference is turned ON using the Temperature Reference Enable bit (TEMPREF). Also note that there is a certain settling time for both Bandgap and Temperature Reference, hence they should be enabled in due time before starting any conversions.

Note: Maximum sampling speed of the internal inputs is 125 ksps.