We also need to limit the bandwidth of the signal. The low stator impedance makes it difficult to simply strap a capacitor across it for low-pass filtering. The inductance in the input circuit destroys the rejection above about 5MHz, but the frequency compensation and limited bandwidth of the preamplifiers rejects any signal above about 1MHz. To limit the magnetic coupling sensitivity we should use a relatively low input impedance amplifier. Hence the choice of a bipolar input amplifier rather than a FET input.
Differential amplifiers preserve the balance of the input signal.
An instrumentation amplifier on a chip is a good choice for a differential amplifier due to the high common-mode rejection, low bandwidth, simplicity of use, low external parts count, and low price. To improve stability and noise immunity I placed both instrumentation amplifiers close together on the PCB and minimized the signal path length. I used linear power supplies to try to limit the nearby noise sources. However, the worst noise problem will always be the reference drive to the rotor. It is physically close to the preamps, it is not affected by the filtering, and has large currents creating magnetic fields.