Simultaneous measurement of pH and ORP

Question

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

MarkThoren · Accepted Answer

Hi Thuanbkhn, 
 It looks like you just replaced the RTD circuit with another measurement channel, which should be fine. 
 How are you determining that there is a 5-10mV offset? Are you driving a very accurate calibration voltage into PH+ and OXY+? Or - are you are saying that two two ORP sensors give slightly different readings? What is the accuracy specification on the sensor? 
 A quick test of the circuit's offset performance would be to simply tie PH+ and OXY+ to V_REF. You should see the offset of the AD8607 (50uV max) plus the offset due to bias current through your 1M resistor (1pA*1M-ohm = 1uV... insignificant). 
 
 Measuring gain error is a bit trickier because you will need a floating calibration voltage such that you can connect its negative terminal to V_REF. 
 -Mark)

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Simultaneous measurement of pH and ORP