I am looking to implement a small coin cell operated circuit that uses a microcontroller, a current source and an op-amp to measure a variable resistance between 10 Ω and 1 MΩ. In order to implement this, I've made a preliminary schematic to outline which parts to use, in no way final or fully specified (resistor values to be calculated / determined).
I have used a PMOS to turn the entire circuit on via a momentary switch, as I would like the circuit to show information on a small OLED screen (not in the schematic) while the button is pressed.
The button press supplies a VDD of 3 V to the MCU, op-amp, and current source. The resistor, specified by R-top and R-bottom, can be within another circuit which is already powered on, and therefore the entire sense circuit should be off and "disconnected" from the resistor (hence the two N-MOSFETs). The things I am wary about in the circuit:
- 10 Ω - 1 MΩ is a very large range; I plan on using a microcontroller with a 10 bit ADC at 3 V or 12 bit ADC at 1.2-1.8 V. Which op-amp configurations should I look at? Which op-amp would be a good fit for this purpose (I am thinking low voltage, rail-to-rail)? Would a log op-amp be better in this case (considering they seem to use many more pins / this circuit should be battery operated)?
- Will the two NMOSFETs properly allow the measurement of the resistor? I am unsure if the drain-source potential difference will be enough to keep them conducting / provide a low resistance path, low enough for a measurement out of the op-amp.
- I am unsure if there are better ways to implement this, or if there are better ICs available specifically for this purpose.