I have been stuck with a problem in my pHmeter expt for almost a month. I searched the net and found many pH circuits and tried many but none are working.I have attached my circuit diagram with this.The problems I face are-1)There is lot of 'noise' present in the output of the TL081. Even if I touch the BNC of the electrode noise can be seen in CRO, 2)The voltage values don't seem to be constant. It keeps on changing with time. If for acid to base I get 800mV change,at the next instant I get 400mV change. What can I possibly do to make my project working??
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I suggest to check analog interface first.
First stage: it would be better if you use a non inverting amplifier with gain > 1.
The schematic isn't complete:
1) how is the op amp power supply ? If it's 5 V the circuit will not work because common mode input range.
2) do you use TL081 or, as in the schematic, or TL082 ? Anyway input current can be not much less than 1 nA. If you use a probe with equivalent resistance in the range of 100 MΩ (quite normal) you'll have a voltage drop (tenth of mV) too high.
Try to look at OPA129 data sheet http://focus.ti.com/docs/prod/folders/print/opa129.html you will find a good amplifier for PH measurement and very useful hints (guard ring) for layout and pcb.
3) circuit with high input resistance can pick up noise if you touch the input. Try to see with oscilloscope if it's main frequency (50 or 60 Hz ?).
