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AD8275 Accuracy Problem


I'm using AD8275 for my project to scale the +/-10V bipolar singal to unioplar 0-4V so that it could be accepted by a 5V ADC. According to the typical appication circuit of the data sheet,i built a minimum one based on this chip.During testing,i found that it worked well for positive signals,but the scaled output was not so accurate as i expected when input was switched to negative range.Can someone help me figure out what the probelem lies in? I attach some measured and calculated value here. 

** The measured reference voltage(VREF) is about 4.1V
** VOUT = VIN / 5 + VREF / 2

  VIN     Measured Value     Calculated Value
+9.5V          3.95V                       3.95V
+7.5V          3.55V                       3.55V
+5.0V          3.05V                       3.05V

  0.0V          2.05V                        2.05V

-5.0V          1.04V                        1.05V
-7.5V          0.54V                        0.55V
-9.5V          0.12V                        0.15V

It's clear that the accuracy is not satisfied when VIN goes to negative.

Thank you in advance.


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