I have a 0.1-4.4ohm rheostat I need to feed to an ADC(assume 10 bit) within a microcontroller. I will do some maths with the counts and eventually set digipots to control some external equipment. The rheostat is on the headlight dimmer switch of a car, and is not an often adjusted item nor is it quickly adjusted. It may take a few hundred mS to go end to end with it at the fastest.
The .1 to 4.4ohm rheostat is a design constraint and cannot be modified. I use the term rheostat as it is just a variable resistor and not a potentiometer/voltage divider.
The circuit will be fed ~12volts as it is utilized within an automotive environment. My current plan is to use a simple automotive grade 5v regulator to run the entire circuit from that way it's all protected.
My plan is to include the rheostat as the ground side resistor in a voltage divider and use a 4700ohm resistor as a current resistor for the high side. This will draw low current and give me a voltage output of 0-4mV based on the .1-4.4ohm rheostat.
As the resolution of a 10bit ADC(assuming 10 bit at this time) is 4.88mV/count, I'll need to amplify the 0-4mV signal to feed to the ADC. My plan here is to use a simple op-amp setup as non-inverting and with a gain of 1001 using a feedback resistor of 1Mohm and tieing the inverting pin to ground via a 1kohm. In theory this gives me a ~0-4volt output I can feed into the ADC to work with. If a gain of 1001 is too high, I could drop lower to 250 or so. The output voltage from the op-amp is not important as I just need enough counts within the ADC to correlate the ADC well with the .1-4.4ohm rheostat position.
I modeled the circuit in LTspice and it appears it would work. I modeled it around an AD8032 but I actually have a MCP603 on hand. The MCP603 was not available within LTspice but if anything, the MCP603 seems better suited? In any event, my breadboard circuit does not work. Only the components shown in the sketch from LTspice are what I have on the breadboard. I'm not sure if I need any caps or anything like that, I'm not sure the real world connections and such an op-amp requires.
Currently, the breadboarded circuit just has an output that is stuck around ~5V. The input into the non-inverting pin doesn't seem to change the state of the output pin. I'm assuming the op-amp is saturated but I'm not sure why or what to do about it- or how I am miss-using it.
I gave the entire scope of my project so that if someone has a better idea/plan they can understand what I'm doing.
If the overall plan is sound, I'm then just looking for suggestions on getting the op-amp up and running with all the supporting components. Other op-amp suggestions are fine, but my requirements are very simple as compared to any other examples I find.
TLDR- I'm looking to use a simple op-amp circuit to amplify a 0-4mV signal to a ~0-4V signal to feed into an ADC. My current circuit isn't working and I'm looking for suggestions on what to try next.