Are you using the multiplexer to read multiple channels? If so, I believe
the sample/hold capacitor is holding the sample voltage from one channel,
then the capacitor needs to be charged to the voltage on the new channel.
This injects current into the source. See
http://ww1.microchip.com/downloads/en/DeviceDoc/33023a.pdf#page=3D429 .
Though the drawing there does not show the multiplexer, SS could be both
the sampling switch and the multiplexer. Note that they specify a maximum
source resistance to guarantee that the capacitor voltage has changed to
the new voltage (for that channel) during the sample time. I've got a
product where some inputs are driven directly by op amps and others are
driven by voltage dividers. The inputs also drive analog switches to an
audio amplifier (the PIC24 measures the RMS voltage of the audio on
several channels while the user is allowed to listen to the audio). The
channels driven by the voltage divider have an audible high frequency tone
due to the charge injection of the sampling capacitor.

Harold


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