30.3.3.5.2 Analog Input Circuit
The analog input circuit is illustrated in the figure below. An analog source connected to an analog input (AINn) is subject to the pin capacitance and input leakage of that pin (represented by IH and IL). When the input is selected, the source must also drive the Sample-Hold capacitor (CIN) through the combined resistance of the input path (represented by RIN). Refer to the Electrical Characteristics section for details on the input characteristics of the ADC.
If a source with high impedance is used, the sampling time can be increased. The required sample time will depend on how long the source needs to charge the CIN capacitor and can be configured using the Sample Duration (SAMPDUR) bit field in the Control E (ADCn.CTRLE) register.