Patrick, indeed the decoupling capacitor is not specified if the internal voltage is switched to output. If you compare this e.g. with FR5969 we specify max up to 100pF while the max load current is up to 10 uA max. So the FR2422 mentions 1mA load current which I think makes it possible to add also 10nF. The point is do you turn off/on VREF between ADC ussage? You always have to consider that the CAP needs to be charged via the internal output buffer. Larger cap means longer time! What concerns me more if you would see this ripple on your supply as well in such case I recommend to decouple your supply properly. Please let me know if this helps.
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