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Offset problem Eval-AD7903

Good morning !

My name is Jordan and i am an engineer student.

I have a question with my evaluation board AD7903. The evaluation board input range is +/-5V. We have an offset of 2,5V next this enter, and then the ADC. When there is no signal the input is 2,5V.

For my application, the input signal range is +/-10V so i add an amplifier OPAMP (AD4898) schematic after this signal with a gain of 0,5 so the output is +/-5V.

But when i link both assembly (OPAMP + Evaluation), the offset of the evaluation board dosen't work (it means my signal is still +/-5V and not -2,5 / 7,5V).

1/ I tried not to link the mass (OPA and Eval board), but there is the electrical grid signal (signal with 50 Hz Frequency).

2/ I link up the mass (OPA and Eval board). when the OPA and the Eval board aren't connected, the functioning is good. But when i linked the output of the OPA (+/-5V) to the evaluation input, the offset of the evaluation doesn't work.

Here is the input schematic of the evaluation board : Vin1- is the OPA signal and next R37 there are an OPA buffer and the ADC.

The Evaluation board : EVAL-AD7903 Evaluation Board | Analog Devices 

Do you have an idea why my OPA signal can't have an offset of 2,5V. What should i do with the mass ?

Is it possible to create my application with the evaluation board or should i need to create a full prototype ?

Thanks for your time.

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