SAM V71 Xplained ULTRA: AFEC pin voltage

[ltiong]

Hi all,

I have a question about AFEC pin voltage of SAM V71 Xplained ULTRA board.

I use pin AFE0_AD2 (labeled as CANRX on board) and AFE0_AD5 (labeled as CANTX on board) for my ADC application.
Both pins show voltage at roughly 3.1V in the beginning.
When I enable-AFEC-channel in program (involves register AFEC_CHER), the pin voltage drops to 2.6V and 2.1V accordingly.
The voltages continue to slightly drop to 2.6V and 2.0V when I set-analog-offset (involves register AFEC_COCR) to both AFEC channels.
And when I set bias-current-control and PGA (involves register AFEC_ACR) to the AFEC channels, the pin voltages show 2.5V and 1.6V.

Does anyone know why do the pin voltages change?
I would like to set the pin voltage at 0V so that it could read the correct voltage readings from sensors connected to the AFEC pins.

Thank you.