8. Grounding and Avoiding Ground Loops
All inputs and outputs are referred to the same ground potential. Ground is also
available at the socket (“GND”) on the rear panel.
The negative supply Terminal (-) is also connected to ground.
To prevent ground loops it is recommended that additional connections are not made
between the sensor cases via the test object. This can be avoided by using sensors
with isolated base or isolating pads which are available as accessory parts.
9. Operation
9.1. Gain Selection
The instrument has four gain ranges of 0, 20, 0 and 60 dB (1, 10, 100 and 1000),
which may be set for each channel individually.
After switching it on, the M208B displays the gain menu (Figure 8).
The selected gain ranges are shown over the respective channel numbers. The flash-
ing value can be changed by the ▲▼ keys. You may select between 0, 20, 0 and
60 dB. Change to another channel by ◄ ►. The active gains are also indicated by
the LEDs over the output sockets:
0 dB: LEDs off
20 dB: orange LED (“20”)
0 dB: green LED (“ 0”)
60 dB: orange and green LED
The gain settings remain stored when you disconnect the M208B from power.
Note: The flashing channel in the gain menu is also the channel connected to the
shared output (see section 7).
9.2. IEPE Indicators
The yellow “IEPE” LED of an amplifier channel will light up when you connect a
sensor to its input provided that the IEPE supply was switched on.
IEPE constant current indication is based on the measurement of the sensor bias
voltage. If this voltage drops below 20 V the LED lights up.
9.3. Overload Indicators
Each channel has a red overload LED. The signal path is monitored for overload
condition at the sensor and at the amplifier output to make sure that signal compo-
nents which do not pass the filter but overload the input stage are properly indicated.
The threshold for the overload indicators is 10 V (peak). At this magnitude the sig-
nal is still undistorted.
10
Figure 8: Gain selection
