I have a resistor divider analog input circuit that has an off-board resistance applied in the form of a sensor. The off-board sensor/resistance can be anywhere from 300 Ω to 12 kΩ.
I see strange behavior on a new board design compared to the same exact circuit on a previous board design. The strange behavior is a sharp drop in ADC reading when reaching a certain resistance to the circuit.
I am simulating different resistance values for the off-board resistor using an Arduino controlling a digital potentiometer (Rtest in the schematic below). I used a voltmeter to measure the voltage drop over the off-board resistor to validate that what I am seeing is related to the circuit and not something to do with the ADC or ADC Ref.
What is even stranger is that if I short the input terminals I2 & G2 together (current path skips over the off-board resistor) and then un-short them, the ADC value goes back to what I would expect. If I power cycle the board, the voltage returns to the higher value until I short the terminals again. The change in voltage drop at the off-board resistor before and after shorting I2 & G2 is ~200 mV
The EXACT same analog input circuit does not demonstrate this behavior on the previous design. The difference between the older design and the newer design is the MCU, but the behavior is seen at the input terminals so the MCU should not be a variable.
There are other changes to the PCB layout (including the power planes) but the analog input changes at such a slow rate that it is basically a stable, DC resistor divider. There is just one ground plane in both the previous working design and the new design.
The chart below shows the ADC values as the potentiometer starts at 1 Ω and increases to 20 kΩ in single-count steps. The dark blue line is the previous board's behavior where you see a clean curve. The light-blue line is the new board design where you can see a sudden drop in the ADC-value curve. The drop in the chart corresponds to Rtest being ~2.5 kΩ.
What is also interesting is that the jump in ADC reading happens also when counting down from 20 kΩ to 1 Ω, but it is at a slightly different potentiometer count.
I am looking for an explanation. I have even removed capacitor C1 and TVS diodes D15, D23, and D20, but still see the same behavior.
Thanks anyone who can help shed some light on this!