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Simultaneous measurement of pH and ORP

Hello, I use CN0326 to measure pH (mV), the result is very good, but I want to measure and add ORP parameters on the same circuit at the same time. i changed Opamp AD8606 (1 channel) to AD8607 (2 channel). Then connect two pH and ORP sensors simultaneously. however i noticed the result when i connected 2 different pH + ORP sensors at the same time than the pH single connection (pH voltage deviation +-5- 10mV). Please explain this phenomenon to me
Thanks a lot