I want to design a circuit which outputs a signal ranging from 0-70 mV using a variable resistor ranging from 80.51 Ohm - 183.05 Ohm and a supply voltage of 12 V. My idea so far has been to make a wheatstone bridge differential amplifier using an instrumentation amplifier with unity gain to subtract a constant offset voltage from the varying input signal. From what I have read, an in-amp is much more preferred than using a single op-amp because of its improved CMMR. I am not sure if it is superior with only unity gain, though.
As seen in the image, I tried drawing this circuit in LTSpice using LT1168 as In-amp. R42 would be the variable resistance, and it will be located on a separate PCA from the rest of the components. Hence, I fear more noise might be picked up on the positive input. I calculated that R43 would either need to be 17313.87 Ohm or 0.85 Ohm i.o.t. have 70 mV difference between the two edge cases for R42. I thought it would be better to go for the first option so as to limit the power dissipation over the resistors.
Hence the offset to be subtracted is formed by voltage divider R21 + R22. With R42 being 183.05 Ohm the output from the In-amp should in theory be 70 mV, and 0 mV when R42 = 80.51 Ohm.
When simulating, however, I only get 500-600 mV as output for any of the cases. It must be said that I am a beginner to LTspice, so I am not sure how accurate the simulation is, but it still gets me concerned. Will it be impossible to achieve such low voltages using this method, assuming that 0.1% resistors and suitable filtering are used? I would gladly accept suggestions for alternative ways of solving the problem as well as any advice for attenuating as much noise as possible.