OP4177 is used as DC differtial amplifier. I have observed that if a positive input is given to Inverting input, the DC gain is marginally different than the input is given to the non inverting terminal. My circuit takes input+28 v to produce -9 volt. But I am getting -9.5V if input is applied to inverting input, and +9 volts when applied to non inverting input. Please help me out to address the issue.

