Sorry it looked like the other questions were put in after I quoted and responded to the original post.That is indeed a fair amount of drift, the rising output. So my first question is how are you measuring this voltage? What device are you using and just where are the measurement points located? I ask this because I have seen folks chase errors and drift that were not there when the measurement system was disconnected. If the rising voltage actually appears in the A/D count then it matters a lot more. Another important question is: Does the indicated voltage increase when the signal connections to the A/D are replaced by a 1K ohm resistor? In the flowmeter circuit is a block with six leads, and no information at all. Is that a passive device with no power conected to it? Or does it contain Some active device that is powered? If it is only an RC filter then I would point at the opto isolator, since it is DC coupled to the A/D through the filter circuit. So there are some questions to answer and checks to do. Those answers will be useful to all readers here.
I put a 10K resistor in parallel with the 2x analog inputs (between the ADC input and ground), and at first I thought it solved the problem... but sure enough it started to come back. Sometimes it comes back on its own, and sometimes me tapping the wire to activate the flow meter makes the problem come back instantly.
You mentioned a block with six leads, are you referring to part U$18? This is labeled as a LTC6993IS6-3 which is a monostable multivibrator. Its also labeled as being powered with 3.3V. Its used to normalize the pulses - any pulse that comes, is converted to a fixed width pulse, this way I can get consistent measurements.
And also like I mentioned previously, I can take a voltage reading on the left side of R76 (between R76 and the monostable multivibrator U$18) and I get 0 volts. I can clearly see the drifted voltage on the right side of R76 (between R76 and the ADC), so it seems the voltage is coming out of the ADC doesn't it?