So I'm trying to measure the signal from a potentiometric sensor, that has a working and reference electrode. Electrically it can be summarized with the picture below. Note that my circuit is intended to be mounted on a PCB and powered by a battery.
Now my problem is that the sensor is left floating. I initially thought that's not a problem with a differential input, but I realized that if it "floats away" it will saturate my internal nodes and kill operation. I know that its not going to drift millions of volts away due to leakage currents, but I am still concerned that it will move too close to one of the rails and distort. I not sure how to estimate how far it will float away other than by physically building it and seeing what happens, hence why I'm looking for a solution beforehand.
I was thinking of the following solutions:
- I can't use a high pass filter because I want to read what is essentially a constant voltage.
- I thought of grounding the reference electrode to my PCB ground, but I'm fairly sure that this defeats the purpose of a differential input. Am I right to assume that the common mode noise would then be transmitted to my signal via ground?
- Do the same thing as above, but via a pull-up/-down resistor. Does adding a resistor change anything? I don't understand why.
- Add a 3rd electrode and connect it to my circuit via a buffer.
I'm getting really confused with this problem and would appreciate any suggestions or pointing me to potential solutions.