A FSR (Force Sensitive Resistor) is a sensor which lowers its resistance when there is force upon it. At idle state (no push force) it has a high resistance (~ MOhms)
Interlink Electronics has provided a guide for using FSRs and explains some interface circuitry there.
Suppose we use figure 9 (schematic below) (Note 'g' actually means 100*N (source: Adafruit's FSR page):
Following the guide, there is a section titled "Adjustable Buffers". Here are my questions:
1- Why should we use an OpAmp? I guess buffer causes the sensor pin not to see Vout's impedance and vice versa. If sensor is in IDLE, R_FSR is ~MOhms and RM pulls the voltage to ground. So, high source impedance of FSR should not matter?
2- What is meant by "adjustments of the output offset and gain"? Exactly, what "offset" and "gain" are we adjusting?
3- In figure 11 (shown below) it is stated : "Typical cal for R5 and the pot are around 10kΩ.". Why R5 is put twice in the schematics? What does it even mean for a potentiometer to have a sigle resistance value?