I'm loosing my mind. I've built a simple ph probe amp board that uses a LMC6001 to slightly amp and buffer the ph probe signal and then send it to a LMC6041 operating as a differential amp with bias to center the +/-0.65V signal centered at 0V to a +/-2.25V signal centered at 2.5V.
Both sections where bread boarded and tested before the PCM was built and both section worked exactly as needed.
This is all background for he simple fact that I have a couple of resistors that are NOT behaving! After having a problem with the final stage not working correctly (output way higher than it should be), I pulled out the LMC6041 and breadboarded the second stage once more to test it again. When that worked exactly as it should have, I decided to pull the opamp out of the bread board so I could ohm out a few points on the breadboard and compare them with the same points on the PCB.
And that's when it got weird. Here is the section of the schematic and the PCB image I am fighting with along with three test points marked on both:
At this point in the testing, I am only testing from A to B, from B to C and from A to C. Everything other than those two resistors (60.4k and 187k) have been removed (specifically, opamp, 100 ohm output resistor and trim pot), so I literally have a simple string of two resistors to test! And this is the weirdness...
From test point A to TP B, I measure the expected 60.4k ohms.
From TP B to TP C, I measure the expected 187k ohms.
But from TP A to TP C (through both resistors in series!), I measure the exact same 187k ohms as before!
I have removed both resistors to verify that there are no shorts between any of the three PCB traces in question (none found visually or with bench meter). I have verified the ohms of each resistor and even replaced the (cheap and I buy them 200 at a time...) and STILL the measurement from TP A to TP C is 187k instead of the expected 247k ohms. And yes, the breadboard DOES measure in the 250k ohms as expected!
I've been staring at this for several hours over the last couple of days. Does anyone else see anything that stands out to them? I'm almost at the point where I am going to make a new PCB and start over. This just doesn't make any sense to me at all.