For signals with frequencies below approximately 1 MHz the use of differential
signaling techniques, also known as balanced input,
has distinct advantages in providing immunity to noise pickup and
crosstalk between channels. This technique depends on the fact that the signal of
interest generates equal but opposite currents on a balanced pair
of wires while noise that is picked up is typically induced equally in both
signal lines. The use of shielded twisted pair cable is key
to insuring that the noise and cross coupling is induced equally in both
signal lines of a cable pair.
The input amplifier (instrumentation amplifier) is designed to amplify the differential (normal mode) signals and attenuate the signals common to both signal lines (common mode), refer to Figure . One measure of the quality of the instrumentation amplifier front-end is the common mode rejection ratio (CMRR). Good instrumentation amplifiers today have typical CMRR ratings of 100dB or more up to 100 KHz or more.
To put this in prospective, a good instrumentation amplifier with an input of an equal amount of normal mode signal and common mode noise would attenuate the noise below one least significant bit (lsb) when the signal was digitized by a 16-bit ADC and thus would not affect the accuracy of the measurement. Bustec analog input modules for signal bandwidths below 1 MHz exclusively use differential inputs to achieve optimum noise immunity.