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

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How to design an OpAmp Circuit for pH Sensor which will be interfaced to PIC.

The PH sensor gives 420 mV for 7pH and -420 mV for 14 pH. I need to design an OpAmp Circuit using 741 or LM358. How can I design the OpAmp Circuit. Further the output of the amplifier will be input to ADC of PIC for measuring the pH.
 

Re: How to design an OpAmp Circuit for pH Sensor which will be interfaced to PIC

While interfacing with the PH probes, the first thing we should take care of its impedance, usually its in 100s of Mohm range. So according to the pH of the solution, its act as battery terminal and generate a potential , ie ~59.16mV per PH ( @25C) . So while interfacing with such a probe, it is better to use a Very high i/p impedance OPAMP. Otherwise there will be a drop in probe's voltage generated, which will give an error. Better go with OPAMP with i/p impedance of Tohm range, which will lead to ultra low input Bias current of fAmps. Say like LMC6041, just configure its as voltage follower, and its output can be connected to LM358, for the scaling.
 


Giving us a parametric specification of your design is completely useless.
A quick search on the Web returned countless results:

https://www.google.com.br/search?q=OpAmp+Circuit+for+pH+Sensor&newwindow=1&tbm=isch
 

No, For me the instructor has given only 741 and LM358N OpAmps and I have to use only these. I have designed Unity gain amplifier using LM358N and this will be interfaced with Non-Inverting Amplifier. The problem is with Non-Inverting Amplifier. I am attaching the Proteus circuit. Please tell me what is wrong in my design. The Amplifier is not vorking properly.
 

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  • OpAmp.rar
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Do not expect people to download your project to then analyze the circuit.
Not all persons have Proteus installed in their machines.

Just attach a clear picture of just the part of the whole circuit which is being discussed.
 

Your sensor is not a conventional pH sensitive glass electrode and that can be seen from the brief specifications you have given. You have given the potential at pH 7 and 14 and we must assume (there are no other evidence) that the pH and potential graph is linear. However, to connect the sensor to an opamp we need more information.
 

Do you have sufficient information about the nature of the pH sensor? Your sensor is not a conventional glass electrode and without this information you cannot go far.
 


In post #1, you mentioned The PH sensor gives 420 mV for 7pH and -420 mV for 14 pH

In post #11, you now say As I already mentioned the pH sensor gives 0 to +420mV for 0 to 7 PH and 0 to -420mV for 7 to 14 pH.

They are not consistent. Yes, this is a glass electrode, you need to figure out whether this is a combination electrode (two connections) or only glass electrode (one connection).

In case this is a single electrode system, you will also need another electrode for reference. For the glass electrode, you will need a FET input based op-amp.

The rest are a matter of details.

- - - Updated - - -

I checked with the amazon site and your electrode is not a combination one; you will need another electrode (either silver or mercury) as a reference.
 

What is not clear. Please mention. It is one electrode. It has BNC connector and the shield connection is grounded.
 

pic.programmer said:
What is not clear. Please mention. It is one electrode. It has BNC connector and the shield connection is grounded.
Click to expand...

You will need another electrode to act as a reference electrode; you need two points to measure a potential difference, right? The shield connection is not part of the sensor, it is just a shield for the high impedance signal.

The you-tube video uses a combination electrode that has two connections from the one sensor. The reference is built in and you will not need another electrode (with the pH sensor mentioned in the video)
 
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Well, you need to buffer the glass electrode potential with a FET-input OP-AMP. The output from this amplifier will be level shifted (zero adjustment) and scaled (60mV/pH) and fed to a display.
 

Yes, I also did that but it doesn't work. The buffer circuit works fine but the level shifting stage doesn't work. I am only testing in Proteus for now. For level shifting I can't use Rail-to-Rail OpAmp. I have to use OpAmp which an use +/- power supply. Also I am not able to find OpAmp with Input Bias Current in femto Amperes.
 

pic.programmer said:
Yes, I also did that but it doesn't work. .
Click to expand...

Can you please be more specific?

I am not able to find OpAmp with Input Bias Current in femto Amperes
Click to expand...

Most common FET input opamps will work with most modern pH electrodes; however, if you want bias currents in the fA range, you will need to hunt for mosfet based opamps.
 

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