For nonratiometric (absolute) measurements where the input signal is not derived from the voltage reference, either use the internal precision voltage reference, or use an external precision reference. Examples of these types of measurements come from sensors such as thermocouples, 20-mA transmitters, and accelerometers.
For ratiometric measurements, where the input signal is derived from the voltage reference, reference noise and drift are canceled by the same ratio of noise and drift within the signal. Ratiometric operation is common with many types of bridge and RTD measurements. For best noise performance, match the reference filter and input filter time constants (see the Section 10.2.1 section for more information). In general, achieve the best ADC signal-to-noise ratio by using large amplitude signals, a large reference voltage, and the highest gain setting possible.