Common Mode - Intab PC-Logger 2100 User Manual

3.3.2 Common Mode

To maintain accuracy in your measurements you have to make sure that no signal
is outside the common mode range. The difference signal that is to be measured
may not have a common voltage with respect to signal ground that exceeds 10V
(see figure below)
A sensor that is left "floating" with respect to signal ground may easily drift
outside this range because of leakage voltages or static electricity.
This will result in, to say the least, unpredictable measurements, but will not
damage the inputs as long as they are within 30 Volts from signal ground. (see
also technical data: Max overvoltage).
(The shield of shielded cables may be connected to signal gnd at one end only!)
Those users who always have "floating" signals should "ground" all negative
inputs by connecting a short jumper between "-" and "Gnd" of the terminals of
each channel.
It should also be pointed out that water must, almost invariably, be considered to
be a conductor.
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INTAB Interface-Teknik AB, Sweden