Driving the analog input of an ADC (Analog-to-Digital cConverter) is not always a simple task. The analog input of many ADCs is a switched capacitor. As such, there are current pulses required at this input that can upset an amplifier that is used to drive it, resulting in amplifier instability and oscillation. The point in time when the ADC sample is actually taken may be anywhere on this oscillation waveform, resulting in noisy and inaccurate capture of data.
There are many technical articles available to help resolve this problem, but the first thing is to be aware of it. The problem can usually be resolved by including a small RC (series resistor followed by a shunt capacitor) between the driving amplifier and the analog input of the ADC. The values of these components will depend upon the ADC sample rate and the use of the ADC. Care needs to be taken not to compromise the signal.