EDIT: Original post was phrased as XY problem.
What I need to do: Determine if power at "Switched power" is being turned on by SW1/FET combination or other power sources not pictured in the diagram below (there are multiple sources). My first thought was to sample the voltage on the switch line - if it is pulled low, the FET is turning on the power. If it is high, it is not.
I'm trying to determine how to measure the voltage V? in the below circuit. It is a high-side switched power circuit using a P-channel FET that is off when SW1 is open.
However, I'm balancing a few competing problems:
1) Resistance into the ADC is too high. By my measure I'd need to keep the sample frequency <100k per second to get a value (computed from answer to Protection of ADC inputs) but the sample rate is 2 Msps on the processor.
2) I can't decrease R1 enough to fix #1 or the voltage V? will drop too much, possibly turning on the FET or putting it into a partially conducting state with high resistance (aka bad).
3) I can't decrease Rfet or it will pass too much current when SW1 is closed.
Note I don't really need an accurate measurement here, I just need to determine if it's GND or some other voltage value. Aka on or off.
Is there a better way to go about this? I saw mention of possibly using an op amp at the sample point.. Obviously the fewer components the better but whatever is required is required.