If the sensors are all individually powered and resistance between the different
input channels is relatively high some of the voltages may come close to the
supply voltage and cause clipping.

Especially if the sensors are fed by a transformer you always have capacitive
coupling from primary to secondary side (and in industrial settings this will
probably also involve being coupled to different phases.

Perhaps you could try to put (even just for testing) a resistor from the negative side
of each channel to the negative supply of your circuit of say 1 Kohm and see if this
brings improvement.

Peter van Hoof



      
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