I have the AD7747 eval board and I am trying to measure the capacitance change of a sensor. I am getting some big discrepancies depending on the excitation level. I am using 5V for Vdd. If I select the Vdd/8 level, my full scale range measured is over 8 pF. However, if I select the Vdd * 3/8 level, my full scale range drops to 1.3 pF. I don't understand how this can be happening. Does this make sense and is there anything that can be done to ensure that I am getting accurate readings?