Guralp Systems CMG-5T Compact - Manual (Page 14)

CMG-5T Compact
signal into the sensor. This can be either a sine wave, a step function or
broad-band noise, depending on your requirements. As well as going to
the sensor, the calibration signal is returned to the digitizer on a full rate
channel (older digitizers used one of the 4 Hz auxiliary (Mux) channels).
The calibration signals and sensor output all travel down the same cable
from the sensor to an analogue input port on the digitizer.
The signal injected into the sensor gives rise to an equivalent
acceleration (EA on the above diagram) which is added to the measured
acceleration to provide the sensor output. Because the injection circuitry
can be a source of noise, a Calibration enable line from the digitizer is
provided which disconnects the calibration circuit when it is not
required. Depending on the factory settings, the Calibration enable line
must be either allowed to float high (+5 to +10 V) or held low (0V,
signal ground) during calibration: this is specified on the sensor's
calibration sheet.
The equivalent acceleration corresponding to 1 V of signal at the
calibration input is measured at the factory and can be found on the
sensor calibration sheet. The calibration sheet for the digitizer
documents the number of counts corresponding to 1 V of signal at each
input port.
The sensor transmits the signal differentially, over two separate lines.
This improves the signal-to-noise ratio by increasing common mode
rejection. If you are not using a Güralp Systems digitizer, the voltage
across the output pins should be halved before converting to
acceleration.
CMG-5T Compact instruments are tuned at the factory to produce 1 V of
output for 1 V input on the calibration channel. For example, a sensor
with an acceleration response of 0.25 V/ms -2 should produce 1 V output
given a 1 V calibration signal, corresponding to 1/0.25 = 4 ms -2 = 0.408 g
of equivalent acceleration.
3.3 Calibrating accelerometers
Both the DM24 digitizer and Scream! software allow direct configuration
and control of any attached Güralp instruments. For full information on
how to use a DM24 series digitizer, please see its own documentation. If
you are using a third-party digitizer, you can still calibrate the
instrument as long as you activate the Calibration enable line correctly
and supply the correct voltages.
1. In Scream!'s main window, right-click on the digitizer's icon and
select Control.... Select the Calibration tab.
2. Under Component, select ALL. The CMG-5T has a single
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