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AD7747 measures different capacitances with different excitation levels?

Question asked by jchang16 on Feb 24, 2016
Latest reply on Mar 2, 2016 by Wa$iM

Hello,

 

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?

 

Thank you!

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