Definite maximum source impedance for ADC module in PIC18F45K22

When the adc initiates a sample it switches in a small capacitor. High source impedance means longer capacitor charging, longer acq time required. A cap at the adc input stores charge from the source and transfers charge to the sampling cap. Less acq time needed, but longer RC time constant on the external circuit.

A voltage divider is needed to measure voltages outside the range of the ADC like measuring 15V supply with a 3.3V ADC. A simple resistive divider is usually fine but low value resistors waste power but high values require longer acq time and can cause other ADC measurement issues. A cap will allow larger values by making the voltage divider impedance seem smaller to the ADC. If you need really higher resister values then it’s best to use an opamp to buffer the divided voltage