Sensor Operational Amplifier – How to Interface a pH Probe


I want to interface a pH probe. The Op-Amp I would like to use is INA128.

My ADC's range is 0 to 5 volts. pH sensor gives a voltage output that is ranged from -0.5 to 0.5 volts, for 0 pH to 14 pH respectively.

Perhaps, I need to give an offset of about 2 volts to Op-Amp and set its gain to 2. If I do that, will negative voltages be substracted from 2 volts?

Best Answer

As far as I know, you need opamps with super low input bias current to interface pH probe. This one has it at 10 nA, that's rather too big. I was able to build one by using opamp with input bias current in femto-amperes range.

You are right about offsetting voltage.

I'd go by finding an opamp which has pH probe interfacing as typical application and implementing the given circuit.

AD549 is a good candidate

LMC6001 -- I have based my design on this

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