Simultaneous measurement of pH and ORP

Hi thuanbkhn,

Can you provide a schematic showing exactly how you have connected the additional channel? A high resolution photograph would also be very helpful.

Note that because of the high impedances involved, all circuitry must be kept very clean. Did you modify the circuit by hand? Is it possible that there is some flux residue or other contamination on the circuit?

-Mark

thank you very much.Please for my suggest

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

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