Function of VREF OUT in ADC10158
Vref is the internally generated reference voltage for the ADC.  The ADC compares the input signal to this reference in order to measure the input signal.

If Vref is noisy, then measured values will also be noisy.  You want Vref to be as clean as possible.

Some regulators (and band gap references) will oscillate if they don't have a capacitor on the output. The regulators use feedback to maintain a stable output voltage.  Since the feedback always lags behind any change, it is possible for the feedback to make things worse rather than better.  Adding an appropriate amount of capacitance slows down changes in the output to something slower than the feedback loop- the loop can then accurately track the output without oscillating.

So, there are two reasons to have the capacitor on Vref:


To reduce the noise on Vref
To prevent possible oscillation on Vref