[MOVED] How to design an OpAmp Circuit for pH Sensor which will be interfaced to PIC

Ok andre_teprom.

Here is my latest circuit. I need to build a sophisticated and precision pH Probe Interfacing Circuit. I can use a 5 USD OpAmp also.

pic.programmer said:

I need to build a sophisticated and precision pH Probe Interfacing Circuit.

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What is the exact mean of that in terms of specifications ?
What is not working so far as expected ?

Concerning to the above OpAmp, once is one designed for high speed signals, for the real circut I would recommend the addition of a small capacitor after the 1st stage in order to avoid excessive oscillation beyond power supply.

It means that price of components are not important. I need a very good pH Probe interfacing circuit.

In simulator it is working. I want to know if the circuit I have posted in #21 is correct. Are there any defects in the circuit ?

What value of capacitor I have to use ? 0.1 uF, 1000 pF ?

pic.programmer said:

What value of capacitor I have to use ? 0.1 uF, 1000 pF ?

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Hard to say, it will depend on how in real-world measured signal behaves. Anyway, when I said capacitor, I actually meant a RC network. You could make some experiments by placing a voltage source ( e.g called "V_noise" ) in series with the input in order to simulate a small oscillation so that could estimate any value to the filter.

Unfortunately I don't understand most of the discussion in this thread.

OPA350 has already typical input current of 500 fA, 10 pA maximum. It will work for pH measurement with standard requirements. You can use OPA129 for guaranteed fA input current. Consider that a unsuitable circuit layout and processing can easily cause much higher leakage currents. Advantage of OPA129 is that the pin layout supports a guard ring design, review the datasheet for layout suggestions.

It's not clear in which regard your level shifter circuit is "not working". The shown circuit can't work in reality because OPA350 voltage supply is limited to 5V. Maybe Proteus knows about OPA350 supply voltage range. You can e.g. use OPA350 with lower supply voltage for the input stage and OPs with higher supply voltage for the remaining circuit. OPA129 is designed for +/-5 to +/-15 V supply.