Dropping voltage at output of opamp

[Minh Thanh]

Homework Statement

I have problem at output of the first opamp . About 3 -4 minutes , the opamp's output voltage decreases by 0.1V . Although I reduced gain of opamp to 2, but the opamp 's output was still reducing 0.1V about 3 -4 minutes .

Homework Equations

The Attempt at a Solution

 

[NascentOxygen]

How many decimal places to the right of the decimal point are displayed on the digital voltmeter you are using to monitor this OP-AMP output voltage?

 

[Minh Thanh]

How many decimal places to the right of the decimal point are displayed on the digital voltmeter you are using to monitor this OP-AMP output voltage?

It is two ..

 

[willem2]

I have problem at output of the first opamp . About 3 -4 minutes , the opamp's output voltage decreases by 0.1V . Although I reduced gain of opamp to 2, but the opamp 's output was still reducing 0.1V about 3 -4 minutes .

And this happens if the input is connected to the ground? If the input is an open circuit something like this could happen, since the input impedance is extremely high and any kind of stray capacitance could form a RC circuit with the input impedance.
If it isn't the input maybe try it without C1

 

[berkeman]

What is connected to the BNC?

 

[Minh Thanh]

What is connected to the BNC?

that is pH probe.

 

[NascentOxygen]

that is pH probe.

And the probe is sitting in mid-air, not immersed in a solution, during the times you observe the OP-AMP output drifting?

 

[CWatters]

What sort of capacitor is C2? Try removing C2 temporarily? The current through R4 is about 120uA so if C2 is leaking 10uA that would reduce the output voltage by 0.1V. Leakage currents for electrolytic capacitors can change when they are new. Might not be this but worth a try.

How stable is the power supply voltage? R6 and R7 add 1.2V to the output...

R7 * 5V/(R6+ R7) = 1.2V

..but if the 5V supply varies so does this value.

 

[Minh Thanh]

What sort of capacitor is C2? Try removing C2 temporarily? The current through R4 is about 120uA so if C2 is leaking 10uA that would reduce the output voltage by 0.1V. Leakage currents for electrolytic capacitors can change when they are new. Might not be this but worth a try.

How stable is the power supply voltage? R6 and R7 add 1.2V to the output...

R7 * 5V/(R6+ R7) = 1.2V

..but if the 5V supply varies so does this value.

I use power supply 5v from Arduino Nano that connected to Laptop by USB

 

[CWatters]

I recommend you monitor the voltage on AOUT (IC1A Pin 1) to see if the 0.1V drop is introduced in the 1st or 2nd stage.

 

[donpacino]

I use power supply 5v from Arduino Nano that connected to Laptop by USB

what else is powered off the arduino?
What is the total current draw of the system?