Intab PC-Logger 3100i How To Use Manual page 18

Connecting Sensors and Transducers (cont.)
The following information also applies to units fitted with jack panels.
The panel is only an extension of the screw terminals. GND is also made
available through two black banana jacks at the logger end.
There are a few basic rules that have to be followed when connecting sensors
and transducers.
Let us first take a look at a block diagram of the inputs. They are differential
and, as can be seen, have no internal connection to signal ground.
Input block diagram
Differential channels measure (as the name implies) the difference in voltage
between the + and - input in relation to signal ground.
Another way of saying this is that each channel measures TWO voltages:
- a) between GND and minus and
- b) between GND and plus.
- The difference is then calculated internally and presented as the channel
value.
By "GROUND" we mean the internal signal ground of the "3100i". It has
nothing to do with safety or system ground of other systems unless you
interconnect them.
Also note that the "3100i" analogue ground is the same electrical point as
the RS-232 signal return AND the 0V of the power supply input
connector.
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INTAB Interface-Teknik AB