The RX built-in A/D converter is a successive approximation type with a sample-and-hold function. The problem may be caused due to the configuration of successive approximation type A/D converters. However, it does not affect A/D conversion results as long as the minimum A/D conversion time and impedances of the permissive signal sources are within the allowable ranges.
In the successive approximation method, the A/D converter stores the electric charge equivalent to input voltage left within the condenser in the sample-and-hold circuit, then digitally converts its value. After the A/D conversion, the charge of the input voltage are left in the sample-and-hold circuit.
Therefore, when switching ports for an A/D conversion (sequentially, as in a scan mode), charge/discharge occurs within the condenser due to potential difference between the newly input voltage and the one left in the condenser within the sample-and-hold circuit. This charge/discharge may seem like noise.
For the minimum A/D conversion time and impedances of the permissive signal source limitation, see the “Electronics Characteristics” section of your user’s manual.