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Design of High-Performance Balanced Audio Interfaces - Part 4

High signal-to-noise ratio is an important goal for most audio systems. However, ac power connections unavoidably create ground voltage differences, magnetic fields, and electric fields. Balanced interfaces, in theory, are totally immune to such interfere
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Isn't this the opposite of what you want?
zapzapouch   6/4/2015 10:08:41 AM
"To ac common-mode voltages, the circuit's input impedances are 1000 or more times the values of R1 and R2, but to differential signals, R1 and R2 have their normal values, making the signal input impedance R1 + R2."


Isn't this the opposite of what you want?  The common-mode impedance should be low to bring the common-mode voltage within range of the op-amp so it can be cancelled out, while the differential impedance should be very high, to avoid degrading the signal.  The whole point of the instrumentation amp is to get a very high input impedance.

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