Error with the gain stage

I said that I decided change the gain stage from a trimpot to a resistor to simply the calculus, but now I made a mistake about the gain stage.

My first version used a trimpot for the offset stage and another one to the gain stage. In this configuration I can adjust the signal with freedom. The code is simple and easy to change. But, I can determine exactly the gain value. This is the point because I decided change the trimpot for a resistor.

Last weekend I was writing a peace of code to make calibration by software. The code is getting complex instead of when I used trimpots to adjust the gain. Today I did some tests with buffer solutions and I used a glass electrode. It applied an offset voltage about 100mV. So, this value with a gain of 5.0 put the signal near of saturation. Therefore, I need fix it.

Another point is with this offset adjust approach generates a signal out from the middle of the ADC scale. So, I lost part of the pH scale.

Now, I am considering to return to trimpot gain to simplify the project, read the voltage reference with an ADC, and put another operational amplifier to change the offset circuit (before to apply the gain).

So, I suggest you don't mount this circuit. I am working on it. There are thing that you discover with a prototype.