When using an oscilloscope to observe the A/D input pin, I can see noise. Why?
The 8-bit and 10-bit A/D converters are successive approximation A/D converters. The phenomenon that you observe is due to the structure of successive approximation A/D converters. A successive approximation A/D converter collects a charge equivalent to the input voltage in the capacitor of a sample & hold circuit, and digitally converts that value. After A/D conversion, the input voltage charge remains in the sample & hold circuit. Consequently, when the port carrying out sequential A/D conversion switches, such as in scanning mode, charge or discharge to the capacitor occurs according to the difference in electric potential between the previous voltage value remaining in the capacitor of the sample & hold circuit and the currently input voltage value. This charge or discharge looks like noise. As long as the minimum A/D conversion time and tolerance level impedance shown in the electrical characteristics are adhered to, there is no significant effect on the conversion results of the A/D converter. Refer to the device's hardware manual for the A/D conversion time (min.) and tolerance level impedance.