Guralp Systems CMG-5T - Manual (Page 10)

CMG-5T
differential outputs with low gain and another pair with high (around
× 10) gain.
Optionally, the second gain block can be configured at the factory to
act as a high-pass filter to remove the DC offset at the output terminal.
If this is the case, the gain at these outputs will be set to × 1 (unity),
with a separate circuit board providing a filter with a corner frequency
of 0.05 Hz (20 s) or 0.025 Hz (40 s). The output offsets of a high-pass
output cannot be zeroed using the DC offset adjustment screws; in any
case, this offset should not be more than ± 1 mV. The high-pass
output is likely to take around five times the time constant of the high-
pass circuit to settle down. This time constant is provided on the
calibration sheet, together with accurately-measured frequency values.
The two pairs of output lines are 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.
For distances up to 10 metres, you can connect the sensor outputs
using balanced, screened twin lines terminated with a high-impedance
differential input amplifier. The sensor outputs have a nominal
impedance of 47 Ω.
The 5T is normally powered directly from a connected Güralp digitizer
through the 10-way connector, although you can use a separate
10 – 36 V DC power supply if you wish. 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 5T unit's
power supply; its filtered outputs provide the ±12 V required to
operate the sensor electronics. The DC–DC converter is protected
against polarity reversal.
The calibration signal and calibration enable inputs are referenced to
the signal ground. If you are using a Güralp digitizer, these lines can
be connected directly to its calibration lines. Otherwise, you will need
to provide a 5 V logic level on the calibration enable input in order to
calibrate the instrument. The signal ground line is used as the return
for both calibration enable and calibration signal lines. See Chapter 3
for more details.
Modifying the sensitivity
This section applies to instruments before serial number T5225 only.
10 Issue E