Input impedance of PIC32 ADC pin
The Reference Manual seems pretty clear on the subject:


  The analog input model of the 10-bit A/D converter is shown in Figure
  17-22. The total acquisition time for the A/D conversion is a function
  of the internal amplifier settling time and the holding capacitor
  charge time. For the A/D converter to meet its specified accuracy, the
  charge holding capacitor (CHOLD) must be allowed to fully charge to
  the voltage level on the analog input pin. The analog output source
  impedance (RS), the interconnect impedance (RIC), and the internal
  sampling switch (RSS) impedance combine to directly affect the time
  required to charge the CHOLD. The combined impedance of the analog
  sources must therefore be small enough to fully charge the holding
  capacitor within the chosen sample time. After the analog input
  channel is selected (changed), this acquisition function must be
  completed prior to starting the conversion. The internal holding
  capacitor will be in a discharged state prior to each sample
  operation.





  An external RC filter is sometimes added for anti-aliasing of the
  input signal. The R component should be selected to ensure that the
  acquisition time requirements are satisfied. Any external components
  connected (via high-impedance) to an analog input pin (capacitor,
  Zener diode, etc.) should have very little leakage current at the pin.


The Datasheet gives a recommended input impedance for the ADC: