>>2941817
Oh damn these sensors are capacitive instead of resistors. No clue why they thought it would be better to define their resistance with a 1kHz sine wave instead of just defining their capacitance. Anyhow, those resistances correspond to a capacitance that ranges from 300nF to 3pF, so I put it in an oscillator with a 1M resistor and an op-amp to buffer it. Also two 10k resistors forming a voltage divider so the other end is reference halfway to ground to minimise DC bias. I’ll use a CMOS 555 timer as a Schmitt inverter to this end.

But now that I know it’s capacitive, that 3pF is getting close to my noise floor. The humidity sensor chip itself is 1mm from a ground plane, making at least 0.3pF of capacitance, and the op-amp’s input capacitance isn’t stated in the datasheet. Because it’s a chopper op-amp, I’m wondering if that means it has some inherent sample+hold capacitance that might mess with me…