According to the pictures in the datasheet, the LTC2508 has up to 500uV pp of noise during large signal settling. Is this really correct?
If yes, where is this noise coming from? Does it disappear after the transient? What's the frequency range?
The noise comes from a variety of sources including the signal source, the ADC driver, external noise sources and the ADC itself. The noise does not disappear after the transient. The frequency range is broadband but the RC filter at the ADC input helps to attenuate it.