11
MAIN UNIT
3
CONNECTIONS
3.3.1
Differential Inputs
There are a two basic requisites that have to be fulfilled when connecting
signals and/or sensors and transducers :
1 - No signal must be left floating with respect to logger signal ground.
2 - System grounds must be connected so that no ground loops are created.
Use a one-point grounding scheme!
The following sections will help in understanding how this can be achieved.
Let us first take a look at a block diagram of the inputs.
Differential channels (inputs) can be considered to be two different inputs:
•
the - input measures the voltage between the GND and
-
•
the + input measures the voltage between the GND and +
The difference between these two voltages is then "calculated" internally.
From the above it must be obvious that just applying a voltage between - and +
cannot possibly work! The amplifier must see signals referenced to GND.
(GND, by the way, is an internal "signal ground" or reference that has nothing
in common with mains protective ground or any other ground that may be
lying around.)
If you have problems in understanding this, it is a good idea to tie the
-
input to
GND by putting the "Diff/Gnd" switch in position "Gnd". The result will be
that the channel will measure the voltage at the + input and then subtract the
voltage at the - input (which is zero volts) and present the correct value.
Just be sure that all your minuses have the same potential so that no
smoke signals are generated.
