9
grounded machine parts. Within earthing systems tran-
sient currents may appear. These transient currents
cause a potential drop across the earthing or grounding
wires. Via the signal input of the amplifier they may
result in a considerable measuring error.
To avoid this, insulated attachment of the transducers is
recommended.
Metra offers several industrial vibration transducers
with insulated mounting base and different insulating
flanges for non-insulated sensors.
A star-shaped grounding network is the ideal solution to
avoid ground loops. Star-shaped means that all
grounding wires of the sensors and the amplifier outputs
are tied to ground at the signal conditioners, without
any transverse connections. In many cases this is more
difficult to realize for the outputs than for the inputs,
because the following measuring equipment may have
single-ended, inputs. If you have the choice to use dif-
ferential inputs, which can be found on many data ac-
quisition boards, you should preferably use them.
2.
4. Amplifier
The instruments of M68 Series have the following
measuring ranges:
Charge mode: 0.1 / 1 / 10 / 100 / 1000 mV/pC
ICP mode:
1 / 10 / 100 / 1000 times
The gain selection switch ”GAIN” has four positions. In
position “Q/10” of the input selection slide switch the
measuring range of all charge ranges is divided by 10.
This may be advantageous for measurement with high
sensitivity transducers or for shock measurement.
)
)
)
)
After connecting a sensor and occasionally after
changing the measuring range, the amplifier needs a
certain settling time because of a short term overload.
Therefore it may take about 30 s, until the output volt-
age is free of DC components.
