Where can I find the ADC voltage reference?

The options available for ADC voltage reference for a device are usually specified in its datasheet under the ADC block diagram as well as in the ADC voltage reference chapter.

Some devices, besides VREF+ and VDD, also provide a Fixed Voltage Reference (FVR) as an option. It has a set voltage independent of VDD and offers selectable output levels of 1.024 V, 2.048 V or 4.096 V. The output of the FVR can be configured to supply a reference voltage to the ADC. Note that the FVR generally has a very limited current output and if you choose to use it externally, it will likely require buffering. Refer to the device data sheet for full details and specifications.

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