Guralp Systems CMG-DM24S12AMS - Manual (Page 64)

CMG-DM24S12AMS Operator's guide
3. Locate the small potentiometer screw head just inside, and turn it
in one direction or another until the output voltage is reading
zero.
4. Replace the pressure cap to keep the instrument's electronics
protected from water and dust.
After the cover is installed, the accelerometer outputs may drift until the
system establishes temperature equilibrium with its environment and
the sensor settles down in its position. If required, the offset adjustment
can be repeated to achieve a better output offset. With experience, it
should be possible to reduce the output level to less than ± 1 mV.
Electrical connections
The CMG-5U sensor has two separate outputs, with low gain and high
gain. The straight-through cable provided will connect the low gain
outputs to the digitiser input (see Appendices). To make use of the high-
gain signal, you will need to make up a cable connecting the high-gain
pins (C and G) of the sensor to the digitiser input pins (J and K). The
sensor outputs have an output impedance of 47 Ω, which is low
compared to the input impedance (1 MΩ) of the digitiser.
The low and high gain output lines are differential outputs balanced
about signal ground so that either differential drive or single-ended
drives of opposite polarity (phase) are available. For a single-ended
drive, the signal ground must be used as the signal return path. You
must not ground any of the active output lines, as this would allow
damaging currents to flow through the output circuits. Also, if single-
ended outputs are used, the positive acceleration outputs must be
interfaced to the recorder.
If you have two analogue ports available, you could make up a three-
way cable which connects both high and low gain outputs to the
DM24S12AMS. You should not attempt to connect both outputs of the
5U to the same digitiser input port.
The sensor is normally powered directly from the digitiser through the
10-way connector, although you can use a separate 10 – 36 V DC power
supply if you wish (again, a custom cable will be required.) The current
consumption from a 12 V supply is about 53 mA. An isolated DC–DC
converter installed inside the sensor housing forms the main part of the
64 Issue B