3. Power Supply and Mass
3.1. Grounding Concept
The inputs and outputs of the signal conditioners are single ended, i.e. asymmetrical. The signal
ground is connected to the outer conductor of the input and output sockets.
The instrument housing is internally connected to ground.
The power supply is signal-referenced, i.e. the negative terminal is connected to ground.
3.2. External Power Supply
The M72 Signal Conditioners are powered by an external DC voltage
A mains plug adapter for 115/230 V AC with 12 V output voltage is supplied with the Signal
Conditioners M72Ax and M72Bx. The power supply socket according to DIN 45323 is located
at the rear of the instruments. Any DC voltage source of 7 V to 28 V DC and around 250 mA
(for M72x1) or 750 mA (for M72x3) can be connected to this socket. The positive supply termi-
nal is connected to the center pin (tip). The ON/OFF switch is located at the rear.
Models M72R1 and M72S1 are powered from the rack cases M72R8 or M72S8 via a 9 pin D-
Sub connector at the rear side of the modules.
3.3. Sources of Errors
3.3.1. Avoiding Ground Loops
Earthing or ground loops are often the reason for measuring errors in multichannel measuring
systems. In most cases you will find a superimposed 50 (60) Hz or 100 (120) Hz voltage on the
measuring signal. One reason for this effect may be that the transducers are ground referenced
not only via their cable at the signal conditioner, but also at the measuring point through their
housing. Vibration transducers are often mounted at grounded machine parts. Within earthing
systems transient 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 consider -
able measuring error.
To avoid this, insulated attachment of the transducers is recommended. Metra offers several in-
dustrial vibration transducers with an 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 subsequent measuring equipment may have sin-
gle-ended inputs. If you have the choice of using differential inputs, which can be found on
many data acquisition boards, you should preferably use them.
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